From experience, I'd be a bit more concerned about structural integrity than hue . . . . . often, when fabrics fade that way, they are also much less strong than they were before the UV exposure that caused the fading . . . .
Sadly, I do not know of a test thay might not destroy the material you are interested in saving ;-}
"Here is your country. Cherish these natural wonders, cherish the natural resources, cherish the history and romance as a sacred heritage, for your children and your children's children. Do not let selfish men or greedy interests skin your country of its beauty, its riches or its romance.” T.R.