Hi Pussyfoot,
A quality paint from a specialist paint shop NOT mixed in a DIY shop like B&Q...so Dulux or Crown etc....
There could be a number of reasons why the paintwork is going yellow quickly, but the most common ones are probably poor preparation where the woodwork has not been cleaned down properly, especially if it is or has been a home of heavy smokers and the nicotine staining is bleeding through the finished coat. Real open fires can cause staining as well. This will be worse if a water based gloss was used and a sealer was not applied first. If the rooms are constantly very warm then the paint tends to yellow quickly.Also if cheap DIY paint has been used and not a good quality trade paint then this wont help either.A good quality water based paint will stay white unlike an oil based paint but will require more coats for a perfect finish.
Dulux specialist shop or Brewers would be an ideal option to buy your paint from....I hope you sort it out, pussyfoot...oh I would also give whatever you're painting a good double underdogs then gloss it...results will look much better. Good luck :-)