It seems you've been exhaustive in your efforts, and I am not a professional cleaner, but if it's just collars and cuffs I would figure it would be wear/stain rather than the fabric "losing its color," especially if it's on various brands. As j mentioned, there have been good threads on properly washing shirts posted, both here and on AskAndy's forum. I find that the toothbrush method works well for me. All my french cuff shirts, particularly white and sea island, are susceptible to stains on the folds of the cuffs and collar (My barrel cuffs from brooks/polo just get tossed in with the regular cycle). I wash within a few days of wear, and before handwashing I flatten out the collar and cuffs, put a little detergent or woolite on the toothbrush and scrub the offending area down. Usually works. Just make sure you keep shirt-scrubbin and tooth-scrubbin brushes distinct. Of course, you may have already reached the end of your patience for this stuff, and if it is a fabric problem, the maker should honor your request for replacing the shirt.