How to simplify the colors in an image

I need to analyze the proportions of different colors used in an image. I plan to use the magic wand with the tolerance set to a low number, then consult the histogram. But before I do this, is there some kind of transform I can give to the (saved copy) of the image as a whole to simplify the colors and reduce the number of individual colors shown? I have a vague recollection of that being possible, but can't recall how it is done.