It's great that the rescue will work with you. I've heard lots of stories of rescues that try to dump a load of blame or a guilt trip on the would-be owner that decides that theirs can't be the forever home for an animal that didn't work out. You don't know how the mix will work until you try it;
With the horses that I have had the chance to observe, most of the more dominant animals were dark, solid colors; most of the light or loudly marked ones were low ranking. If I were to name the animals by rank in the various "herds" that I handled, the names at the top of the list would usually be dark colored animals. Some of the lower ranking animals might be darker colors, too, but most of the time, the louder, the lower. Not to say there weren't exceptions (Latte is a palomino, and she has always ranked pretty high in any group she's been in). Color doesn't create the personality, but personality creates rank - in a group of half a dozen bays, they don't take turns being the boss, the most dominant personality wins, but in a pasture with half a dozen bays and an Appaloosa, all of the bays may gang up and universally pick on the spotty one.
The relationship between my two is rather odd - we joke that Sunny (a light chestnut) is Latte's punching bag. As much as she picks on her, you'd think Latte despised her, but Latte is the one whose brain falls out when they lose sight of each other. We used to think that Latte was just being a snot to Sunny, but lately, I've been questioning that. I don't think Sunny is brazen enough to challenge Latte outright, but I wonder if she isn't "dissing" Latte just enough that Latte feels she needs to prove her position again, and again, and again. Certainly, that's how Sunny acts when ridden - she frequently needs to be reminded who calls the shots. When these two lived at the farm where I used to work, Latte had an annoying and dangerous habit of running up to challenge any other horse that was ridden in "her" pasture, and Sunny was usually two steps behind, like the "wingman" in a fighter plane strafing run. I'm not sure where Sunny would rank, without Latte to lead or get in her way.