What a poetic theory, Penth. I like it. It's like all the rest of the color of Ennis' life runs out of him when Jack dies. He goes from the brown of the earth to the gray of the rock beneath.
I still see it as Ennis going directly from the bus station/diner to the post office to the phone booth all on the same day. When you watched it again, Katherine, did you get this sense, too? I can see where he'd go to Lightning Flat on a different day - he'd have to look up the Twists and arrange a visit with them beforehand after all. But those three other things seem to take place in succession to me in the movie, anyway.
Barb, I've always assumed they happened on different days. Either of us could be right, I guess (let's check to see if the shirt under the jacket changes to find out for sure!).
OK, so I just watched the movie for the first time in two months, obsessively keeping an eye on shirts throughout. I have reached the conclusion that you're both right. The gray jacket is about Jack dying, but it's also about what Ennis is feeling. Here's why:
I think the color of the shirts and jackets, their style, and even the number of layers that Ennis and Jack wear almost ALWAYS mean something. Others have commented on some of this stuff elsewhere, particularly in that long-ago color thread -- that came early in my Brokeback career and it was too much detail for me to absorb at that point, so I only skimmed it. Hope this isn't too repetitive.
Now up on Brokeback, Ennis usually wears a tan coat when he's pulling away from Jack somehow -- the day they meet, when he's shy, the day after the first tent scene, when he's freaked out. He usually wears just a shirt when he's moving toward Jack -- working together to set up camp, shooting the elk. Usually the shirt is Ennis' color, tan. There are some exceptions (and a few times I forgot to notice), but that's the usual pattern. In the first tent scene, Ennis is wearing the tan jacket, but this is a transition moment for Ennis. Jack is ready to get closer to Ennis, so he takes HIS jacket off. In TS2, they've both moved another step toward each other: Ennis in just a shirt, and Jack in no shirt at all, still a step ahead of Ennis. Neither is wearing a shirt in the happy tussle -- they're equally happily into each other. Ennis and Jack are both wearing just a shirt in the angry tussle, obviously, but by the time they get back to town ther're both in jackets, separated.
During their four years apart, Ennis wears blue -- denim jacket or print shirt -- in just about every scene; a bright blue print shirt, for example, when he's spreading tar and thinking about Jack (I remember someone pointing out his blue cap in the sledding scene). Jack wears blue, too. Again, exceptions, such as Ennis in the bedroom with Alma -- but then SHE'S wearing a light blue nightgown, as if a pale substitute for Jack.
For the reunion, Ennis wears his nice blue-striped shirt. Jack is wearing a jacket -- Ennis is actually a step ahead of Jack, for a change. In the motel, no shirts for either, they're totally together. When Ennis packs for camping, he throws in all blue-plaid shirts. When they first get to the campsite: no clothes at all! Can't get much more together than that! But in the "you know it could be like this always" scene, Ennis is back in the tan jacket, pulling away. (As others have noted, in the Earl flashback, nine-year-old Ennis is wearing the famous plaid-and-denim pairing).
The pattern continues most of the time until the divorce. Ennis wears a denim jacket when away from Jack but presumably thinking about him, such as when throwing hay off the truck. Jack wears a purple shirt when he's looking for his blue parka and talking to Lureen (others have pointed out that red is Lureen's color, and that this shirt indicates a Jack/Lureen mix. Sometimes Alma wears a dull red, too, such as when she spots Ennis and Jack in the stairwell, and at Thanksgiving dinner). Both Jack and Ennis wear beige down jackets at some point; I have no idea what that means.
Then the post-divorce scene. Ennis wears that really heavy tan corduroy jacket -- a thick layer between them. Jack is wearing (what? black leather jacket?). The big news here is that FROM THIS POINT ON, Jack almost always wears gray or black. As if Jack metaphorically died at that point (a la Jake's comment).
Ennis wears blue when he's alone or with Cassie (missing Jack) and tan when he's with Jack (pulling away). He's confused: always inclined to movel in the opposite direction. (Cassie, incidentally, wears a blue denim jacket on their date with Alma Jr.) At the lake scene, Jack is wearing a tan parka (indicating he still has a connection with Ennis? note the lining is even darker brown) over a muddy-grayish-blue shirt (death). Ennis is in the heavy tan jacket, STILL pulling away. In the dozy embrace, both are wearing heavy jackets. Your guess is as good as mine: maybe a harbinger of their lifelong separation?
Then the pie/Cassie scene. As we've noted, FROM THIS POINT ON, Ennis never wears tan again. Like Jack earlier, he now always always always wears gray, the jacket buttoned almost all the way up over a light shirt. This is why I agree with Penthesilea that Jack literally died around this time and, though still unaware of it, Ennis metaphorically mourns with his clothing.
In the last scene with Alma Jr., Ennis is wearing the gray jacket over a light DENIM shirt. Mourning, but keeping Jack close to his heart.
And when he opens his closet shrine, in addition to the famous shirts there's a blue-plaid shirt hanging from closet pole (and a red-plaid shirt -- not sure why; because both wives sometimes wear red, maybe this indicates Ennis has come to think of himself and Jack as life partners? I can't imagine Ennis actually wearing the red plaid).
What do you think? I may be reaching a bit for some of these, I know. But I'm sure there's something there.
UPDATE: I just copied most of this post onto the color coordination thread over at CT. It seemed very on-topic there, and a bit OT here.