SurfaceShader Semantic COLOR1 not working

I am having trouble compiling a SurfaceShader when adding the COLOR1 Semantic to the v2f struct.

struct appdata 
{
	float4 vertex : POSITION;
	float4 tangent : TANGENT;
	float3 normal : NORMAL;
	float2 texcoord : TEXCOORD0;
	float2 texcoord1 : TEXCOORD1;
	float2 texcoord2 : TEXCOORD2;	
	float4 color0 : COLOR0;
	float4 color1 : COLOR1;
};

Vertex program ‘tessvert_surf’: unknown input semantics COLOR/1
(on d3d11)

wiki.unity3d.com says it should be so, yet it’s not.

What’s the correct Semantic, if any exists?

You describe trying to add COLOR1 to the v2f struct, but your code snippet shows you ate trying to add COLOR1 to the appdata struct…

If you read the page you linked to closely, you’ll see that only a single COLOR semantic is supported in appdata - which is the vertex colour attribute passed from Unity to the vertex shader. (Consult the first acceptable fields list on that page)

You can use COLOR1 when passing data from vertex to fragment shaders, but you wouldn’t use the appdata structure for that - you’d use the v2f struct.