reducing color depth ?

I've gotten this effect many times but now I can't remember how to do it. I seem to remember being able to call up a window with a slider that you could use to reduce the number of colors in a picture. it was useful for screen printing because you could reduce the image to the number of colors you were printing with. although i do seem to remember the name of the effect being counter intuitive, and stumbling upon it completely by accident, actually I'm not even sure it was an effect i think it was in the adjust or mode menu. please help me out, it's driving me mad.

ha! i figured it out... indexed color...

check it out

it's good for that grungy punk rock look.