Ennis' pale and mixed colors clothing indicate roots: he olny knew one kind of life, the ranch life, and also he only knew Wyoming and his quiet life (living with a wife or living alone and working, nothing else). That kind of colors indicate introspection and lack to adapting himself to changes and also fears and need to go unnoticed.
Jack's solid colors clothing indicate capacity to adapting himself to changes (first BBM, then Ennis, then Texas, then Lureen, then the "once in a while".....).He was like a chameleon, he believed in everything he did, and the strong colors of his clothing showing his cheerful, open and changeable character.
My sweet MPJ NASTY, I also noticed the symbolism of Jack's red shirt. SS tell us that Ennis wore his best shirt to see Jack in the reunion scene, so probably Jack did the same: he wanted to be nice for Ennis. But is so meaningful the image Alma saw through her window: A nice man in red shirt leaning on a red truck; was it like a provocation? Like a challenge? Red color means love, passion and, as you said, temptation, but for a lot of ancestralt cultures it have always meant prohibition: something red is something forbidden!!! And if you consider BBM as their Heaven on Earth, their love as the forbidden apple, and Aguirre as the God's angel who threw them out from paradise, you clearly can see their reunion like something forbidden, and because of that Jack was also "something forbidden" to Ennis, and for Alma Jack in red was like something diabolical or wicked who was going to destroy her marriage.
So for me Jack in red is a very powerful image meaning love and of course passion, but also a clear warning: If you catch something forbidden you know what you'll get.........,punishment, pain?