My experience, and this is with waterfowl, the bright "face flash" of an unpainted face and movement are the two things that catch the birds attention. I would imagine plains game react the same.
My outfitter and PH told me that subdued clothing and being still/slow are what is necessary for plains game. Following their advice I wore subdued green clothing. I would say he proved it when we had a young Kudu bull come to within 20 yards of us while we were waiting for a herd bull to clear the bush for a shot. The youngster never detected us. Five days later I shot a gembok bull out of a herd that grazed from 300 yards up to 50 yards of us where we were hidden behind a bush, and you all know how spooky gembok are especially a herd of the darn things. We had to let them get that close due to there being acacia shrubs in the line of sight.
Both incidents were the result of being very, very still and quiet, and being positioned in favorable wind. By the way, we were almost trampled by a pack of mongoose while waiting on the gemsbok herd, the little guys went past us close enough that I could have touched one with the muzzle of the rifle.
Listen to your outfitter and PH, they know the score. Good luck on your hunt and be sure to post a report with photos.
I'm really sensitive to heat, esp. humidity...I always avoid dark greens or browns because they soak up sunlight/heat...A mid-green works best for me