My experience, is the heather gray turns "pink" when they fade from washing and in the sun. My wife does not hang them in the sun, but dries them in the laundry room hanging upside down. She does partially dry them in the dryer.
The charcoal's, Gerry Davis brand, are going on the end of their 3rd season, and show no signs of fading. I do like the GDavis pants and they seam to be holding up well. Yes I would buy them again.
