Since no one who really understands all that has answered, I'll tell you what I know.

If you go to the NASA Mars Exploration site, they will explain that the pictures are taken several times through various filters. Then the exposures can be compared to see which minerals stand out in the various wavelengths.

Apparently they have published some unfiltered ones. I think they publish the information necessary for folks to combine and create color images at home, too, but I'm not sure of that.

AND, it just doesn't look very colorful out there. Times Square it ain't.

Sorry for such a sketchy answer. But the NASA Mars site will explain all, and has links to explain more.

Arf