I dry mine in a dehydrator at 105F. Color is usually maintained. The key to very fine powder is the peppers have to be dry dry DRY before grinding them.
I know this is an old thread, but I can't believe no one has said this.. Do not grind your peppers in the house. The dust will cause everyone in the house to gag, choke, and tear up something fierce. It is a painful experience. Yes I know from experience.
Also I set my dehydrator to 160 degrees and did not discolor my red habaneros. dehydrated them in about five hours.