Did some tests. Most of the characters work fine with a dark BG, expect for the Chinese girl, the vampire chick, and the robot. The Vampire chick and the robot have mostly mid-range to darker colors, so they really start disappearing on a dark BG. The Chinese chick has an intensely blue dress, with dark gray hair. Finding a color for her that doesn't make her dress or her hair vanish is tough, but here's some that I came up with...
Thoughts?
LT: The darker shades for the first set of images definitely work better. I'm gonna sound anal, but with the 32x32 limit, I'm definitely not feeling body shot images with BGM where something reaches the edge; it just makes it look bootleg how the background color doesn't envelop the image. Maybe try to resize the images for those so they're 30 pixels tall maximum, if the resize looked good. It might be too hard to get some of these looking good on any sort of solid background color, but maybe you'll hit paydirt with other colors.
Designating them 1-10 with 1-5 being the top row and 6-10 being the bottom:
I'd take 3, 7, 8 & 9 as is, definitely. They look awesome. 1 and 5, I'd take with a different background color than that brown shade. 2, 4, 5 & 10 are the clearly the toughest to really hit a solid BG color for, as you've already found out. Sorry for taking so long in my reply, Coop. Thanks for the effort on these so far!