IMO any halter that would look good on a sorrel will suit a red roan since they have a solid red face. So the consensus seems to be that both green and blue are good choices. I think simple black suits every colour.
When Ace was a weanling & we showed him at Nationals, we showed him in a black rolled leather halter. That is what we generally use on the stallions-- it is refined , yet masculine looking (in our opinion.)
I don't know how to post pictures on here-- have a picture on our wall - taken at the 2002 Nationals-- has Ace in it with his rolled leather halter.