A simple vertex color shader using CG.

VertexColor.shader

Shader "VertexColour"
{
	SubShader
	{
		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#include "UnityCG.cginc"
     
			struct v2f
			{
				float4 pos : SV_POSITION;
				float3 color : COLOR0;
			};
			
			v2f vert (appdata_full v)
			{
			  v2f o;
			  o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
			  o.color = v.color;
			  return o;
			}
			
			half4 frag (v2f i) : COLOR
			{
			  return half4 (i.color, 1);
			}
			ENDCG
		}
	}
  Fallback "VertexLit"
}