It seems the color fading of human hairpiece hair due to sunshine can be successfully prevented to a large extent. One product that seems to work very well is Redken Sunshape Sun Milk. It contains a strong SPF sunscreen, is a nice styling aid, and smells nice too. Control experiments of one unit with, and one unit without, have not been tried here, but it seems to protect the hair from any noticeable sun fading. There may be similar products from other makers.

Synthetic hair generally never fades.