I've done it a number of times...by (Login qb432l)
In my graphics tutorial (QBG) in the lesson on color, there's a color mixing console the user can use to experiment with how RGB intensities interact with one another. In Animax! (versions 1 & 2) you can mix 12 customs colors. Finally, in my new 256-color graphics/animation utility (GFX256) you will have the same capability, except with 220 attributes (it will be finished within a week or so). All these consoles use mouse-controlled RBG sliders, show individual intensity values, and display the mix in a window as it changes.