BJara ! nice to see you. I understand your feelings and agree, somewhat.
This is one of the reasons that Dark dust bags are provided for use ! Because protection sprays need to be reapplied a few times per year, it is the bag owner's choice and responsibility, It is possible to darken a bag dramatically by handling it with lotioned hands, environmental smog, laying it on the floor, or any number of other things.OP didn't give specifics or post photos or tell us if she tried to cleanse the bag, applied anything to it, etc. Too much guesswork here. I doubt Turquoise was field tested for three months of continuous exposure but I have seen a very faded blue LP that the owner set in her sunny car window routinely.(Blue bleaches out more easily than any other color so teals and turqs could show more green over time). LP may be unaware of the need to state the obvious. Everyone is familiar with tan bags (of any brand) turning darker, or 'honeyed'. It's natural. Same thing happens to colors.
Lambskin in particular ~ think of how a lamb would sunburn with all that natural lanolin on it's skin and no wool for protection !
Anyway, This set me on a search and I will be testing UV protections for naked leather.(being shipped now.)
If it works nicely, I'll pass along details in the care thread. No way am I giving up my Turq Crocos.