Hm, I'm pretty sure palladium is the one it is alloyed with most of the time. Stuff like nickle is quite rare and it being alloyed with platinum to become white is even rarer. And like I said, if it has a rhodium coating (which also isn't a common thing), it means the white gold is very poor quality- a reason to not buy it.
Palladium is white and stays white. They are making jewelry out of it by it self now.