I heard a rumor that the color of the core does matter. From what I heard, in the light pure silverish does the best, then blueish, then purple/red, then mixed colors, but, I have a silverish 2100 that did horrible and a purple/red 1700 that does really well. Truthfully, I think it is just better to get a good stepping and week then to judge by the color.
As for the color of the chips, I think that is random. I asked AMD once, and they said it was random. Sorry, but that e-mail is long gone. Anyways, I have seen green and brown of the same week and stepping before. I think the only difference is that the green ones are harder to to bridge mod.
-0cer