I don't have an RS but a BBB white skin Espree boy. I've had him for about 3 years now and yes he is mellowing to a lovely cream. I think the main thing is to keep them out of the direct sunlight as much as possible, but not obsessively. So if I'm doing pictures, yes he might be in direct sun, but not riding in the car or his normal place in the house. If I'm going to be carrying several dolls around at a festival, I usually have them sitting in a large basket, and I drape a piece of white cotton or muslin over the basket as a sun shield.
when I first got him, and got a Kiki-chan78 face up:
last year at DollAKon: