OK, I just watched if for the SECOND time in two months (also in two days). Silk, I saw a lot of exceptions to my theory above. There are times when Ennis and Jack wear jackets even though they clearly are growing closer (the "Wha?" scene, the "You may be a sinner" scene). Ennis wears a grayish jacket on one earlier occasion, in the condiment aisle scene. He wears his jean jacket with Jack once, too: the time when Jack suggests that Ennis move to Texas. Jack wears red and other colors of shirts a few times, and although in the first half of the movie he usually wears bright or navy blue, he also sometimes wears a sort of muddy blue that could almost be called gray. In the last scene, Ennis isn't wearing a denim shirt, unfortunately, though it is light blue, almost solid, with a a faint blue plaid pattern. So there are quite a few exceptions.
Still, the vast majority of clothing choices DO match the theory. So I think there's gotta be something there. Unlike buckets or coffee pots, which only have to show up now and then to make their point, Ennis and Jack (almost) always have to be wearing clothes of some kind. Maybe it would look odd if they followed TOO rigid a pattern.
Anyway, everybody can just decide for themselves what they think.