i dabbled in Painter many years ago (v7, i believe), so maybe it has this, but i don't remember seeing it. the only place i've ever seen it is in the painting module in the compositing program Combustion. there's a little palette you can mix colors in and save. its really neat, and the quickest, most useful way to get the range of colors you'd actually get using real paint.
artrage already has that SUPER cool way to make that little window that you drag around the screen to capture colors you want, which is amazing! it'd be awesome, though to just have a little blank palette down there that you could squirt colors into and mix, then save.
but, maybe it does that already and i haven't found it! ops: if so, please let me know!
other than that, i'd say add watercolors, and maybe make the keyboard shortcuts customizable, then you guys can retire cuz your program will be perfect!!
if it's not fun, what's the point?