Ooooh! Interesting one! How did I manage to miss this thread?!
Well for me, I've always thought that Jack's outfit/hat is darker/black to balance out the tan/brown shades that are Ennis. I'd never really attached much symbolism to it really, but I don't think there's anything intended about Jack being the "bad guy".
Anyhow, being a web designing geek, and being obsessed with colour schemes (I always obsess over colour schemes for new sites, and I'm infamous on the web design forum I hang around on for maintaining a list of the top colour scheming software - LOL!), I decided to do a scientific/non-scientific analysis for each character by pasting various images of them from throughout the film into one image for each character, and then setting the software to 'random' to see what it came up with for each of them.
...so here's a colour scheme for each of them, and one added on the bottom that's for Brokeback, from the movie poster.