I grey my palette down a lot. This tends to add more of a look of realism. Most colorists to my mind over saturate their colors which give their pages a heightened, fantasy look. While I like the way the saturated colors catch the eye, I can't seem to use them for my own work. It's like I am physically unable to do it or something.
My best advice is to copy colors that you like in your favorite photos. Sample and use those.