TECHNICAL FIELD [0001] The present invention relates to an optical fiber ribbon, an optical fiber cable, and a method of manufacturing an optical fiber ribbon. The present application claims priority based on Japanese Patent Application No. 2016-194548 filed on September 30, 2016 in Japan, the contents of which are incorporated herein by reference. BACKGROUND ART [0002] Conventionally, an optical fiber ribbon as disclosed in Patent Document 1 is known. This optical fiber ribbon is formed by connecting a plurality of optical fiber colored core wires to each other with connectors. Each of the optical fiber colored core wires includes a bare optical fiber, a primary layer covering the bare optical fiber, a secondary layer covering the primary layer, and a colored layer disposed outside the secondary layer. By forming the primary layer of a soft material having a small Young's modulus, an external force applied to the bare optical fiber can be relieved, and an increase in transmission loss of light due to the external force can be prevented. In addition, by forming the secondary layer disposed outside the primary layer of a hard material having a large Young's modulus, the bare optical fiber and the primary layer can be protected from an external force. Furthermore, the colored layer is colored for discriminating the optical fiber colored core wires from each other. PRIOR ART DOCUMENTS PATENT DOCUMENTS [0003] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2013-134477 DISCLOSURE OF INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0004] By the way, the above primary layer may be formed of a UV curable resin. In this case, when the UV curable resin to be the primary layer is cured by irradiation with UV light, reliability of the optical fiber ribbon may be deteriorated due to insufficient curing. [0005] The present invention has been achieved in view of the above circumstances, and an object thereof is to secure reliability of an optical fiber ribbon in which a primary layer is formed of a UV curable resin. SUMMARY OF THE INVENTION [0006] In order to solve the above problem, an optical fiber ribbon according to a first aspect of the present invention is an optical fiber ribbon formed by connecting a plurality of optical fiber colored core wires to each other with connectors formed of a UV curable resin, in which each of the optical fiber colored core wires includes: a bare optical fiber; a primary layer formed of a UV curable resin covering the bare optical fiber; a secondary layer formed of a UV curable resin covering the primary layer; and a colored layer disposed outside the secondary layer and formed of a colored UV curable resin, and the primary layer has a Ycung's modulus of 75% or more with respect to a saturated Young's modulus of the primary layer. EFFECTS OF THE INVENTION [0007] According to the above aspect of the present invention, it is possible to secure reliability of the optical fiber ribbon in which the primary layer is formed of a UV curable resin. BRIEF DESCRIPTION OF THE DRAWINGS [0008] Fig. 1 is a cross-sectional view of an optical fiber colored core wire according to the present embodiment. Fig. 2 is a view for explaining a configuration of an optical fiber ribbon according to the present embodiment. Fig. 3 is a cross-sectional view of an optical fiber cable according to the present embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS [0009] Hereinafter, configurations of an optical fiber ribbon and an optical fiber cable according to the present embodiment will be described with reference to Figs. 1 to 3. Note that in Figs. 1 to 3, the scale is appropriately changed in order to make il possible to recognize the shape of each constituent member. Fig. 1 is a cross-sectional view of an optical fiber colored core wire according to the present embodiment. As illustrated in Fig. 1, an optical fiber colored core wire 1 includes a bare optical fiber 2, a primary layer 3, a secondary layer 4, and a colored layer 5. [0010] The bare optical fiber 2 is formed of, for example, quartz-based glass and transmits light. The bare optical fiber 2 has, for example, a mode field diameter (MFD) of 8.2 to 9.6 pm in light having a wavelength of 1310 nm. The primary layer 3 is formed of a UV curable resin and covers the bare optical fiber 2. The secondary layer 4 is formed of a UV curable resin and covers the primary layer 3. The colored layer 5 is formed of a colored UV curable resin and is disposed outside the primary layer 3 and the secondary layer 4. Note that specific materials of the UV curable resins to be the primary layer 3, the secondary layer 4, and the colored layer 5 may be the same as or different from each other. Examples of the UV curable resins include an acrylate resin. [0011] Fig. 2 is a view for explaining a configuration of an optical fiber ribbon 51 including the optical fiber colored core wire 1 of Fig. 1. As illustrated in Fig. 2, the optical fiber ribbon 51 is formed by connecting a plurality of the optical fiber colored core wires 1 to each other with connectors 115 disposed at intervals. More specifically, the plurality of optical fiber colored core wires 1 is arranged, and the adjacent optical fiber colored core wires I are connected to each other by the connectors 115. Hereinafter, a direction in which the optical fiber colored core wire 1 extends is referred to as a longitudinal direction, and a direction in which the plurality of optical fiber colored core wires is arranged is referred to as a width direction. The width direction is orthogonal to the longitudinal direction. The connectors 115 are disposed in the longitudinal direction of the optical fiber colored core wire 1 at regular intervals. With respect to a position of a connector 115 connecting adjacent optical fiber colored core wires 1 to each other, a connector 115 connecting one of the adjacent optical fiber colored core wires 1 to another optical fiber colored core wires 1 adjacent thereto is disposed at a position shifted in the longitudinal direction. In this manner, the connectors 115 are disposed in a staggered manner in the width direction and the longitudinal direction. [0012] By forming the optical fiber ribbon 51 as illustrated in Fig. 2, the optical fiber ribbon 51 can be rolled to form a tubular shape or folded. Therefore, the plurality of optical fiber colored core wires 1 can be bundled at high density. Furthermore, the adjacent optical fiber colored core wires 1 are connected to each other by the connectors 115 disposed at intervals in the longitudinal direction. Therefore, by peeling some of the connectors 115, a specific optical fiber colored core wire 1 can be easily taken out. The connector 115 is formed of a UV curable resin. In the example of Fig. 2, the optical fiber ribbon 51 includes four optical fiber colored core wires 1, but the optical fiber ribbon 51 may be formed using five or more optical fiber colored core wires 1. [0013] The optical fiber ribbon 51 can be used for a loose tube cable, a slot type cable, a ribbon type center tube cable, a wrapping tube cable, a micro duct cable, and the like. The micro duct cable is a type of loose tube cable and is obtained by packing optical fibers at high density in a loose tube having a small diameter. Due to such a structure, relatively strong side pressure acts on the optical fiber colored core wire 1 in the loose tube cable, and transmission loss of light may increase due to microbending. [0014] In order to suppress transmission loss of light when side pressure acts and to improve a microbending resistance characteristic, it is effective to form the secondary layer 4 or the colored ayer 5 of a hard material and to form the primary layer 3 of a soft material. In this manner, by making the primary layer 3 in contact with the bare optical fiber 2 soft and making the secondary layer 4 or the colored layer 5 located outside the primary layer 3 hard, the bare optical fiber 2 can be effectively protected from an external force. The secondary layer 4 or the colored layer 5 preferably has a Young's modulus of 700 MPa or more and 1400 MPa or less, for example. [0015] Fig. 3 is a diagram illustrating an example of an optical fiber cable 50 using the optical fiber ribbon 51. The optical fiber cable 50 includes a plurality of the optical fiber ribbons 51, a binding material 53, a wrapping tube 54, a cylindrical sheath 55, a pair of tension-resisting members 56, and a pair of tear cords 57. [0016] The binding material 53 bundles the optical fiber ribbons 51. The wrapping tube 54 covers the optical fiber ribbons 51 bundled by the binding material 53. The sheath 55 coats the optical fiber ribbons 51 together with the wrapping tube 54. The pair of tension-resisting members 56 is buried in the sheath 55. The pair of tear cords 57 is buried in a position close to an inner peripheral surface in the sheath 55. A marker protrusion 58 protrudes from an outer peripheral surface of the sheath 55 at an outside of each of positions where the pair of tear cords 57 is disposed. The marker protrusion 58 is formed along the tear cord 57 and indicates a position in which the tear cord 57 is buried. Note that the optical fiber cable 50 does not need to include the wrapping tube 54, the tension-resisting member 56, the tear cord 57, or the marker protrusion 58. In addition, the optical fber cable 50 may include only one optical fiber ribbon 51. [0017] Next, a process of manufacturing the optical fiber cable 50 will be described. [0018] In order to manufacture the optical fiber cable 50, first, a bare wire forming step is performed. In the bare wire forming step, the bare optical fiber 2 is formed. The bare optical fiber 2 is drawn out from a glass preform heated to, for example, about 2000°C, and is formed to have a desired outer diameter. The outer diameter of the bare optical fiber 2 is, for example, about several hundred pm. Next, a primary layer forming step is performed. In the primary layer forming step, a UV curable resin to be the primary layer 3 is applied around the bare optical fiber 2. Thereafter, the applied UV curable resin is irradiated with UV light to be cured to form the primary layer 3. Next, a secondary layer forming step is performed. In the secondary layer forming step, a UV curable resin to be the secondary layer 4 is applied around the primary layer 3. Thereafter, the applied UV curable resin is irradiated with UV light to be cured to form the secondary layer 4. Note that by applying a UV curable resin to be the primary layer 3 around the bare optical fiber 2, subsequently applying a UV curable resin to be the secondary layer 4 onto the UV curable resin to be the primary layer 3, and irradiating the UV curable resins with UV light, the primary layer 3 and the secondary layer 4 may be cured collectively. That is, the primary layer forming step and the secondary layer forming step may be performed simultaneously. [0019] Next, a colored layer forming step is performed. In the colored layer forming step, a UV curable resin to be the colored layer 5 is applied around the secondary layer 4. Thereafter, the applied UV curable resin is irradiated with UV light to be cured to form the colored layer 5. As a result, the optical fiber colored core wire 1 is obtained. Next, a ribbon forming step is performed. In the ribbon forming step, a UV curable resin to be the connector 115 is applied to the plurality of optical fiber colored core wires 1 at a plurality of positions at intervals in the longitudinal direction. Thereafter, the applied UV curable resin is irradiated with UV light to be cured to form the connector 115. As a result, the plurality of optical fiber colored core wires I is connected to each other to obtain the optical fiber ribbon 51. Next, the optical fiber ribbon 51 is housed in the sheath 55 to obtain the optical fiber cable 50. [0020] In this manner, in the process of manufacturing the optical fiber ribbon 51, irradiation with UV light is performed a plurality of times. Here, the inventor of the present application has found that curing of the primary layer 3 may progress even in a step after the primary layer forming step. Specifically, in a case where curing of the primary layer 3 in the primary layer fonning step is insufficient, when the primary layer 3 is irradiated with UV light in a subsequent step, the UV light which has passed through the secondary layer 4 and the colored layer 5 is absorbed by the primary layer 3, and curing of the primary layer 3 progresses. [0021] When such a phenomenon occurs, the Young's modulus of the primary layer 3 may become harder than a desired range, an action of relaxing an external force by the primary layer 3 may become insufficient, and transmission loss of light may thereby increase. In addition, due to insufficient curing of the primary layer 3, when water comes into contact with the optical fiber ribbon 51,the primary layer 3 may be peeled off from the bare optical fiber 2, or water bubbles may be interposed between the primary layer 3 and the bare optical fiber 2 to cause side pressure to act on the bare optical fiber 2. [0022] Focusing on the above problem, transmission loss of light, reliability, and the like of the optical fiber cable 50 have been verified, and a result thereof will be described with reference to Table 1. Note that each of examples illustrated in Table 1 uses the optical fiber colored core wire 1 having MFD of 9.1 pm for light having a wavelength of 1310 nm, an outer diameter of the bare optical fiber 2 of 125 pm, an outer diameter of the primary layer 3 of 19C um, an outer diameter of the secondary layer 4 of 239 um, and an outer diameter of the colored layer 5 of 252 urn. This optical fiber colored core wire 1 conforms to G652D or G657A1 defined by, for example, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). Note that the outer diameters of the primary layer 3, the secondary layer 4, and the colored layer 5 are design values, and the actual dimensions thereof have an error of about ± 3 pm. In addition, the dimensions and the like are mere examples, and the results obtained from the examples illustrated in Table 1 can also be applied to the optical fiber colored core wire 1 in which the dimensions and MDF are changed from the above values. [0023] (Table 1) ~~ CM t-; (-; ,-; ,-; u z £ 3 CM CO Tl- ° ° ° ° OLU odd _ > CO a" £^ b£ +J n, (TJ T— CO at r> en i- ,-, --) <-, a O Z. £ £ in in (TJ a a ° a ^LU odd _ >t>J SS SS 3S +-> a, *r o CM 2"£ tDCOCSOT-CMO.-r,^ 5 g- ^ *V -s. ,- CM CO ^ U it EISCSS-00020 ^UJ o d ^ _ >— cS cS cS -5 « °° 3 ,— ^LU odd to iS ^S ^S I I I I „ CO — O |3££°°°°00 U Q Q Q If} ^ e£ &^ u oo co r^ -^COCJlCJlcMtOCOOv^^ ^\-^\r-i-^cM^^ = c2[^p0000'00 LJ d d d -t ^ ^s ^ I I I I „ r^ io co -^(oi^oo^cMcoir^^ ?\"-s.\CMOOO;=;E EtocogooodOO LJ o o o ci £S as ye u T U3 CM — r^r^-cooOcaciicM^,^, ^■^■^VOOT-^CM^^ !£££°°°°°° LJ d d d CJ se =? s= I „ in r» in -£incar-cn,*T(or--^^ 3-^-^'^'3-oooiTt |CMC5C;°°°°00 LJ d d d ~ ^ ^ ^ „ 1— U3 O >J c it i ii ^ -. ^ ^ S ° £ooco00000 LJ d d d o o o tfl ETLi_ "ti^ " |-S ^.E |-S S43 [0024] (Definition) The "primary Young's modulus" in Table 1 indicates a Young's modulus of the primary layer 3 in each state during a process of manufacturing the optical fiber cable 50. For example, the "primary Young's modulus before coloring" indicates the Young's modulus of the primary layer 3 after the secondary layer forming step. The "primary Young's modulus after coloring" indicates the Young's modulus of the primary layer 3 after the colored layer forming step. The "primary Young's modulus after ribbonization" indicates the Young's modulus of the primary layer 3 after the ribbon forming step. The Young's modulus of the primary layer 3 is determined by applying shear stress to the primary layer 3 in a state where the bare optical fiber 2 is fixed, measuring a strain, and drawing a stress-strain curve. [0025] For example, focusing on the primary Young's modulus of Example 1, the primary Young's modulus is 0.50 MPa before coloring, 0.60 MPa after coloring, and 0.63 MPa after ribbonization. The rise in the Young's modulus of the primary layer 3 as the process progresses in this manner means that curing of the primary layer 3 progresses by UV light which has passed through the secondary layer 4 and the colored layer 5. This tendency is common to Examples 1 to 6 and Comparative Examples 1 to 5. [0026] Note that Table 1 illustrates the degree of curing together with each numerical value of primary Young's modulus. The degree of curing is a ratio of the primary Young's modulus to a numerical value of saturated primary Young's modulus described later. For example, in Example 1, the primary Young's modulus before coloring is 0.50 MPa, and the saturated primary Young's modulus is 0.70 MPa. At this time, the degree of curing before coloring in Example 1 can be calculated as 0.50 ■*■ 0.70 a 0.71 (71%). Therefore, in the column of "primary Young's modulus before coloring" in Example 1, a numerical value of 71 % meaning the degree of curing is also written together with the numerical value of 0.50 MPa. [0027] The "saturated primary Young's modulus" in Table 1 indicates the saturated Young's modulus of the primary layer 3. More specifically, the "saturated primary Young's modulus" in Table 1 indicates the Young's modulus of the primary layer 3 in a case of performing irradiation with UV light including a wavelength contributing to a curing reaction in an amount sufficient for completely curing the primary layer 3 in a state where a UV curable resin to be the primary layer 3 is applied to the bare optical fiber 2. For example, in the present Example, when a UV curable resin to be the primary layer 3 was iradiated with UV light having a center wavelength of about 365 nm at 1 J/cm2, even if the curable resin was further irradiated with UV light, the Young's modulus of the primary layer 3 did not rise. This state is defined as a state in which the primary layer 3 has been completely cured. In addition, the above "degree of curing" is calculated based on this saturated Young's modulus, and therefore is an index indicating how much the primary layer 3 is cured in each state. [0028] The "microbending characteristic" in Table 1 indicates stability of light transmission of the optical fiber colored core wire 1 against side pressure. Specifically, the "microbending characteristic" indicates a value obtained by measuring the magnitude of transmission loss of light passing through the bare optical fiber 2 under conditions that a tension is 1 N, a sandpaper is #360, a length is 600 m, and a bobbin size is