I haven't tried any of this, but I'm going to tell you what I'm listening to on my DVD.
I just got the baking arts DVD. This guy's instructions to make this kind of cakes:
Those cakes are covered in rolled chocolate. He is talking about not using white chocolate because the colors can't be that vivid. He uses coating chocolate (white or dark). He's also saying that the other reason not to use white chocolate is because it contains cocoa butter (which has a lower melting point). The other two ingredients in his chocolate modeling paste are corn syrup and glycerin.
And then, he adds just the regular blue color from Americolor. And he talks about what you might have heard about the color not being good for chocolate. He's saying it's not true, and that if you check the ingredients for the color, it's corn syrup with a dye, and that's not going to seize your chocolate.
Haven't checked though, if that's true, but according to him, using the gel is fine. It's actually his preferred coloring.
So...cocoa butter can't have vivid colors (still_learning might be right)...gel colors might be OK to paint/color your chocolate, or so Richard says in his video.