Even if you hand pick and paint every color in your image, if you rely only on the luminosity slider, you'll have the same problems. You see this luminosity slider reliance a lot, especially in the skin tones of the work of beginning painters.
Bam! Hit the nail on the head there, friend. I'm not using the luminosity slider, but I am using opacity mixing a lot, and I'm getting dull grays when I mix. The solution, then, is to pick the correct color between the two I'm trying to mix and add that in, instead of over-mixing everything. Looks like I'm going to be experimenting with the color picker for a while...