Does white gold have a slight yellowish tinit to it or is it just white?

on the inside of my engagement ring it says 18k plat .51 what does that mean? the description of my engagement ring is 18 k white gold ive had my ring for a little over a month and the ring is getting a yellow gold tint to it in certain light i can't even see it but in other lights it really shows the yellow to me at least i thought maybe it was the platinum dipping coming off but i don't really know much about jewelry if my fiance finds out he spent the money he spent on a crap ring he's going to flip the diamond is 5.5 k and it is beautiful and very clear i believe the diamond is good he got the ring from kays did he get snowed on this should we go to kays and raise hell? because believe me we will