Have you noticed that after she's in the water that she is licking herself? I have noticed that if you don't dry them off, they lick themselves dry and over time it will discolor

I have never notice her licking herself in that area after water play, actually she seems to play and just air dries. My other yellows don't get this discoloration after water play.

Get some malacetic spray (pump bottle) from your vet and spray that area. Once you get rid of it your good, it won't go away on it's own. It's caused from too much moisture that doesn't get fully dry.