18k WG is 75% yellow gold, 25% lesser metals (to make it white).
(Same ratio as 18k RG, which is why RG sometimes yellows over time, too.)
(18k YG's 75% gold, 25% other metals to make it harder, but also less yellow.)
(I've actually found that 14k WG yellows less--b/c there's only 54% yellow gold in it).
Depending on skin chemistry, some people will see their WG turn faster, and some not for years and years. But it's natural--in a way, it's validation that there's a lot of real gold in there, as there's supposed to be!