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.