Multiple instances of same part in assembly, can I make them different colors?

I have an assembly and I have the same part twice, and I would like them to be two different colors in that assembly.

I am having trouble in that if I change the color of one instance in the assembly, it is changing the color in the base part instead of just the one instance, so the other instance changes color too, which is not what I want. Am I doing something wrong or is this not possible?