Hey guys can you chroma key out multiple colors? I guess I'm going for a pleasantville-ish effect to where I can make a single object (not just one color) in color while leaving the rest of the screen desaturated. Any tips?

Yes you can you need top use several layers in FCP

Yep, several layers and the matte filter. Selective color replacement in Apple's Color may be a better solution, although I'm still learning that program.

Yes, Color is the way to do it, however Apple's 3 wheel color corrector can do secondary color correction/grading/replacement/whatever-you-want-to-call-it.
This article will help you understand the plug-in in general and also covers color replacment.

this will also help

to do multiple colors in a clip you obviously would have to add multiple color correction filters and probably have to use masks and layers to get the desired effect. Could get hairy. But hey, no one said color grading was easy :P