Which color is this?
There's a list of colors, or only so many possible colors. You only want one of those colors in the list/that is possible.
You might not be aware of the list/which are possible, so if someone asks you this unexpectedly, it's OK for you ask "What possible colors are there?"
What color is this?
There's no list, so you're not expecting the reply to conform to anything.
You also can ask this if you don't know the name of a color at all.
Even if you don't know the name, but there's only so many possible colors (e.g. you're asking the color of 1 specific crayon out of set of 64), you might still use which.