Is there a way to communicate between the gameobject and the shader?

Is there a way to set some sort of flag on your gameobject so that custom HLSL shader code and read it and change its properties accordingly?

for example,

a gameobject could tell the material/shader to change its color multiplier to red.

or more specifically to my needs, I just need it to pass an index number to a property in my custom shader code.

Use Material.SetColor and access your shader property through that.