If you just need a constant material color:
Just add a new float4 constant to the shader and store the material color in that constant.
Then multiply it with the ouput of the pixel shader.

return color * IN.shade * mat_color;

If the material color varies bake it into the diffuse color texture or add a second texture to the shader, sample it like the diffuse texture and multiply with the output.

If you just need a constant material color:
Just add a new float4 constant to the shader and store the material color in that constant.
Then multiply it with the ouput of the pixel shader.

return color * IN.shade * mat_color;

If the material color varies bake it into the diffuse color texture or add a second texture to the shader and sample it like the diffuse texture and multiply with the output.

If you just need a constant material color:

Just add a new float4 constant to the shader and store the material color in that constant.

Then multiply it with the ouput of the pixel shader.

return color * IN.shade * mat_color;

 

If the material color varies bake it into the diffuse color or add another texture to the shader sample it like the diffuse texture and multiply with the output.