I always thought Ennis had 3 shirts. A brown plaid, the famous one, and a third that looked a lot like the famous one only it was a gray or gray/blue. If you watch Jack as he is washing clothes or when the boys are cutting wood, it looks like Jack is washing the gray/blue one and Ennis is wearing the gray/blue one.
I don't know. Maybe my eyes are gone all fuzzy. You guys have a look see and tell me.
Ok, I watched Ennis's shirts carefully last night and I'm certain he only has two: the "famous" blue/white one that he wears in SNIT and on their last day on Brokeback and the brown/orange one that he changes into after they fight, which is the same one that he's wearing during FNIT and the "morning after" (you can see the edge of the collar quite clearly both in the tent and when he's saddling up the horse as Jack comes out of the tent).
One thing I did think, though -
if we're assuming that SNIT happens the day after FNIT, then at some point between the "one shot thing" conversation and the scene where Ennis is sitting at the fire trying to decide whether he's going to go to Jack or not, he changes his shirt. Logical, since he'd been wearing it during FNIT so it was probably, um, sweaty, but I bet that particular clothes change was about the most awkward ever

'Course, it could be another clue that SNIT did not happen the night after FNIT - guess I'll need to go watch the "one shot thing" scene again to see if I can see which shirt Ennis is wearing at that point. All in the name of clearing this particular question up, you understand

Jack, I think, is washing the shirt that Ennis finds in the closet at the end (the whiter one). I think Ennis is wearing the orange/brown/checked shirt while Jack is washing Ennis' lighter colored shirt. I only counted two Ennis shirts when the boys were up on BBM. But heck, it'll give me an excuse to watch the first 40 minutes again (like I need one!).