Hi Unhappychick,
Assuming that you are referring to your internal paintwork:
It is highly likely that you have been using a solvent based paint which is prone to discolour (especially Brilliant White). Discolouration is "speeded up" if the room suffers from a lack of natural light.
I would recommend that you try a good quality trade Acrylic Gloss such as Leyland High Gloss. I have used this product many times to obtain an excellent professional finish.
Check here for your nearest supplier:
http://www.leyland-paints.co.uk/index.php
Good luck
toneless