Penthesilea, Latjoreme, hi! Thanks for the welcome, I am enjoing this thread.
The bus station - like you I always found it strange that he was in a bus station of all places. Especially considering that he seemed to have his favourite bar in town. It was the mention of men with black and white hats buying tickets (I have not spotted them yet, look forward to yet another detail to take in) that gave me this idea, as, like you, I have always been of the opinion that by that time Ennis was ready for some change. Probably going somewhere metaphorically rather than geographically for the time being (eg giving more of his time to Jack), but who knows, perhaps he was finally starting to give more consideration to that cow and calf operation?
Back to the hats:
the very white hat with Bobby in the tractor and while selling tractors - somebody mentioned, very astutely, that the very white hats were the straight men (Jimbo and his friends, the tractor buyers). In this case, Jack is trying to conform, and to give the image of a straight man. Ennis is just off-white, ie trying desperately to look straight and conform to straight principles and way of life , to convince himself that he is staight really, because he was quite sure that's what he should be; while Jack, more confortable with his sexuality, just lies blatantly to others (very white, not off-white) for convenience and for his own safety but he does not lie to himself (he never gives up on the black hat)
Black hat at the 'You're late' fishin trip. This is a trip when he does not mention anything about living together. He is abiding by the rules that Ennis has set for their relationship, and trying to make the most of what he gets without asking for more. But he arrived before Ennis although he has such a long drive to do, he is VERY keen to see his man and keep him, whatever the conditions.
The Reunion and the After Divorce are scenes where he goes to Ennis with the express intention of arranging a future together - these are almost marriage hats. I don't think he intended to make another attempt during the 'come to Texas' fishin trip, he just happens to mention it because the conversation leads this way. But it is the only other scene apart from Reunion and Divorce when he mentions living together, and he wears a brown hat, so there must be some link. Perhaps to show that living with Ennis was always on his mind? His answer to Ennis comes out so spontaneously, he was just 'thinkin out loud' ie this was in his mind all the time and just escaped from his mouth.