Did you be sure to get the skrews thru the holes, and then tighten both properly using a hexagonal key and skrew driver?? And are you sure that you didnt cross thread as you tightened the screws, or that you didnt tighten it entirely on one side before starting to tighten the other?? All these things are basic hardware knowledge, all could cause the saddle to become damaged, and none are the responsibility of wintec.
Also, did you use the proper screws, and were they coated with the protective lubricant??
I cant see that the gullet could change color after use. I could understand that the color could fade, but that doesnt add up to what you have described. If you used a gullet of a different brand, or one of the wintecs that arent recomended for your saddle, it is negligence on your part.
Ive used many wintec products. My Isabel has been changed to both extemes, from the extra wide white my mare uses, down to a yellow to suit a tbred dressage horse for state, and then back up again to white for my girl, without any damage to the saddle.