I noticed the variations of hat colors from the start, but I didn't see any significance. Just that they wore what goes good with their clothes and boots. Even cowboys have to be coordinated...that means hat, belt, boots must match. It also means they wore what they had if they couldn't afford a big wardrobe of hats for every "outfit". That fit Ennis, but Jack, well, he could afford it, and we see several different styles and colors. Symbolism at work here? Perhaps. Every movie can be scrutinized for hidden meanings (the eating scene in Tom Jones comes to mind); sexual inuendos are probably the most obvious, but do we see that sort of thing in BBM? So at this point, I take that stand that the hats don't mean anything more than what you see.
And by the way, the only hat I was interested in (and it's really not the hat, but what's under it that got my interest) was on the head of the guy dancing with the dark haired lady in the blue dress doing the twrills in the dance scene that opens the benefit story. There's a great shot of just his face framed between a couple in the foreground, over the right shoulder of Jack. I noticed this guy immediately and replay this scene over and over and over, etc. Just thought you might want to know. (Don't know how to insert the "wink face" here, so please visually insert it for me)...Doug