Well, he did wear white hats at the office. Still had solid color shirts. Not really sure what you mean by looked different.
I trhink it's because in the BBM scenes and in the fishing trips, he's a bit more on the blue/violet side of the color spectrum, rather than on the red/yellow side of it. That's probably what I sense as the 'difference'.
Well, here are all the colors of Jack's shirts:
Wyoming Brokeback Mountain - Blue
Reunion - Red
It could be like this camp out - Tan
2nd camp out - Purple
Ennis' after divorce - Black
3rd camp out when Ennis is all paranoid - Gray
Last camp out - Gray
TexasMeets Laureen - Gray
Baby Born - Blue
Selling machine - Gray
Blue Parka - Purple
Jack & Bobby on tractor - Gray
Thanksgiving - Gray
Dances with LaShawn - Dark Maroon
So, as you can see, except for 3 times, all shirt colors were in the blue family (blue, gray, purple and maroon). However, the 3 times they were not in the blue family, he wore them in Wyoming.
Interesting side note. There were 7 times in Texas and 7 times in Wyoming.