That is an interesting insight, Lee. You are right that black, white and grey are mentioned prominently. For instance, in the third paragraph, Aguirre's ashtray was mentioned as well as his hair, the color of ash, while the window admitted a triangle of white light. On the mountain, the trees are massed in slabs of somber malachite. The sheep are the color of dirty laundry. In contrast, the dawn is described as orange and pale green (sounds strange, but I have seen dawns like that!) In fact, light and fire are often yellow...the boys sit in the yellow light of a kerosene lantern, while the flames of the campfire are described as yellow silk sashes.
The next day, the colors are enlivened...there is a green washcloth, copper jean rivets, and a lavender sky. And AP actually says that Ennis felt he could paw the white out of the moon! It's almost like that scene in the Wizard of Oz where everything turns from black & white to color!!
Throughout the days on the mountain and their descent, colors are mentioned, but there is a premonition in the worn paint brands on the sheep, the hailstorm, and the early arrival of the white snow. Not much color appears until the reunion, when a soiled orange chair is mentioned in the room at the Motel Siesta. Whites, blacks, and blues predominate, with red represented by the periodic mention of blood. The river is tea-colored and the willows are ochre-branched.
Suddenly, when Ennis finds the shirts, the color red is everywhere, not just used as punctuation as it was before (the ruddy chunks of light, the cherry cake). It was a "gushing nosebleed...when Jack...had slammed Ennis's nose hard with his knee. He had stanched the blood, which was everywhere, all over both of them, with his shirtsleeve, but the stanching hadn't held...." Yes, we know, Ennis's stanching of his breaking heart hadn't held, and he had lashed out in anger and wild despair, at the ministering angel that was Jack. Then, it's back to the browns and greys, the dirty horseblankets, the sopping coffee filter. But in Ennis's dreams, the colors are so bright, they are described as "lurid."