On my old maple basses, it is the finish getting darker and on really old ones, turning yellow. I have not refinished a bass to see if the whiteness returns when the finish is striped off, and it is quite different on old Gibsons verses, say Alembics, which used much different finishes - yelloowing verses just darkening. I greatly prefer the darkened and yellowed finishes over the new. It looks great and means the wood is aged, so it probably sounds a bit sweeter - All in my head? Are all things that way, though? Just an opinion on my part, really.