Write shader color property from shader and read from Unity

I need to be able to modify a shader color property from within the shader itself and then be able to read that color from unity. I have a native texture that is set to Default usage which means I cant read the pixel color data from the texture so I want to be able to read a specific pixel or set of pixel colors from the shader itself. Is this possible?

The reason I come to ask is that I cant so far seem to get the color out of the shader, it doesnt seem like the color gets set from inside that shader or its ignored.