Most of their white gold is rhodium plated and I believe it is still their policy to re-plate it for the lifetime of the piece. In the past some pieces like the Century Ring have been released without plating, and I haven't heard about any yellowing, but the piece was already a more titanium-like satin-finished grey when it was new.
As far the rose gold, the patina it gets, another name for the microscopic scratches from daily wear, causes it to soften into more of a yellow color. Tiffany will polish rose gold items and they look new afterward.
If you like white or rose gold better than yellow I think you should buy them. If you're impartial you should probably stick to yellow gold, as it's the least likely to change color appearance over time. If you really like the white, maybe consider saving up for platinum jewelry.
Hope that helps.
Tiffany white gold is the only gold I can wear without an allergic reaction. I don't know if they plate it, but they don't seem to use nickel which is what I'm allergic too. I'm guessing they use another white metal maybe palladium which means it would stay white, and not yellow.
I'v had my white gold clover key for years and it has not yellowed.
Tiffany does clean and polish free for life. But I've never heard of them re-plating for free. In fact, I've never heard anyone needing to re-plate at all. Best to call and find out.
I don't know if Tiffany will charge a fee to replate their WG or RG pieces. Their RG pieces do lighten up in colour over time. I own several Tiffany RG pieces purchased at different times. My DBTY which I have been wearing 247 for 2.5 years have lighten over time and my newer RG piece has more red in it. I hope this makes sense.