oil based 1 or twice goes yellow in what seams to be a very short tme
water based 2 to 4 times (better undercoat untill it looks white then top coat) if you try to topcoat yellow paint work it never seams to look white
water based takes lots of painting to look good but does not yellow ive painted our bedroom 3 years after id painted it the first time yes(dont let your otherhalf go shopping in a diy store with cheap wall paper they tend to find wall paper they like)and then you get to remove the old wall paper and redo the whole room you done not that long ago
after applying only 1 top coat it did could not even see any differance so does stay white
oh and dont skimp on the sanding i now use a battery multi tool with sanding pad a 80 grit disk and dont push hard to give a good key