Well, if they're living on the plains and stuff, all out in the open and exposed, they wouldn't wear bright colours, especially if they're still tribal (Being plains people) they wouldn't be all "Hey, you know what is great to wear with all this green and brown around us?" "Purple, bright red, pink, maybe a splash of blue, and white?" "YES. That is what we should wear." Though, it all depends on what sorta...system of they have going. If they're all nomadic and stuff, or have like...the Mongols with some sort of Semi-Permenant residence thing going on. Still, bright colours might be a bad idea of they want to be all unnoticed and not targets, but that could always be their hunting gear.
Also. facepaint. >_> I love the idea of facepaint.