Some vintage wraps change color (fade, yellow, turn greenish, etc.) due to exposure to UV light, some oxidize, and some just change color overall, apparently due to some sort of internal chemical reactions.
I'm not sure which is the case here with your snare but Ludwig didn't offer a Yellow Pearl nor do I recall it being available elsewhere or as an aftermarket wrap (at least not until relatively recently).
In the case of my WMP club date set the yellowing is even on both the drums (bass and small tom) and has a beautiful butterscotch tone to it. The fact that both drums match each other so well and that the fading is so even overall on both drums leads me to believe that this particular batch of WMP was predestined to do this somehow by one of its ingredients, or, perhaps, how it was made. Not intentionally intended to yellow of course, but I have seen several other mid-60's WMP drums that look like this so maybe a batch or two of wrap had something unique about it that caused this to happen. Other WMP drums have been known to evenly take on a very slight greenish tint in much the same way. The only downside is that it makes it tough to find drums that match and my added-on floor tom is much whiter than the other two.
Now if I could just find a matching snare....Hmmm? Might need to look into a new Ludwig in that aged onyx Pearl.