Why is my custom shader rendering completely transparent?

Question

I'm currently trying to learn how to write shaders, so I'm getting my feet wet with Unity's Shaderlab format. One of the first things I tried was to write a surface shader that supports vertex colors being assigned from code. Unfortunately, whenever I try to apply the shader to my object, it just renders the object completely transparent.

Shader "Custom/SVCA" {
    Properties {
        _Color ("Color", Color) = (1, 1, 1, 1)
        _MainTex ("Base (RGB) Trans (A)", 2D) = "white" { }
    }

    SubShader {
        Tags {
            "Queue"="Transparent"
            "RenderType"="Transparent"
        }

        CGPROGRAM
        #pragma surface surf Lambert alpha

        struct Input {
            float2 uv_MainTex;
            float4 color;
        };

        sampler2D _MainTex;
        fixed4 _Color;

        void surf(Input IN, inout SurfaceOutput o) {
            o.Albedo = tex2D(_MainTex, IN.uv_MainTex).rgb * IN.color.rgb * _Color.rgb;
            o.Alpha = tex2D(_MainTex, IN.uv_MainTex).a * IN.color.a * _Color.a;
            o.Specular = 0.2;
            o.Gloss = 1.0;
        }
        ENDCG
    }

    FallBack "Diffuse", 1
}

What is causing this issue, and how can I fix it?

Answer 1

Apparently all that was going wrong was happening when I declared the Input structure here:

struct Input {
    float2 uv_MainTex;
    float4 color;
};

All that was going wrong was going wrong due to the fact that I forgot to explicitly specify the COLOR semantic on this line:

float4 color;

With the COLOR semantic specified, the above line of code should look like this:

float4 color : COLOR;

Always remember to specify your semantics when they're required.