I tend to like blue and green in G10. Though with Ti, I prefer it to be natural, or some shade of gray. Micarta I like black or white.
That said, I have only a couple of green scaled or covered knives: A Spyderco Worker and a GEC Mako with smooth green bone covers. They were supposed to be "blue" but these were green.
I generally do NOT like red. But I have a red LionSteel SR-2a that I really like. On that knife, for me at least, the red works. It will very likely be my only red knife, ever. I do not even have a red SAK anymore.
On traditional folders, though, I like wood, usually ebony, though I'm trying a few others. Smooth bone is my favorite cover material, and I really like white or ivory colored bone, but colored bone (green or blue) will also work for me.