Do you use the colored ones under corresponding colored inserts. i.e red under red, green under green, or do you just use the white?
Yes, I color match LED to insert. If there's no good match, white does fine, just make sure it's not a real cool or warm white or else it can make the colors go off a bit. Also, be real picky about the differences between amber and orange, because you want to keep those looking true.
For white inserts, there are several different kinds of white LEDs. I like a cool white in my inserts as they pop more, but warm white looks more like the original incandescents. It's really a preference thing when it comes to that.