Newbie question - darkening paint w/out black
So I see often on here the advice that you shouldn't mix paints with black, and that art students are discouraged from buying paints that are anything but red, blue and yellow.
Today I was actually painting with acrylics for the first time in years, and so, remembering that blue is darker in value, used that to help me with the shadows. It was certainly a lesson, lol. :P
But what if you want a colour that's darker than the blue paint allows? And how do you stop colours from being overly bright and saturated? Is using white considered acceptable, or is it discouraged just like black?