I regularly go to look at jobs for customers where their woodwork has gone yellow, when it was originally white. Often it is a newish home and sometimes an older place that has been redecorated in the past. The reason for yellowing is due to the contractor using oil based paint.
A few years ago oil based paint was reformulated to meet VOC emissions and the result was that yellowing occurs quickly. VOC or volatile organic compounds are released as the paint dries, a bit like solvents. They can cause problems like headaches etc. There are also suggestions that they are carcinogens. People sometimes say 'How long will it take to yellow?' and my reply is, well technically from the moment its dry, but certainly within six months is will have started to be noticeable. Some manufacturers claim that their paint takes longer to yellow.
The yellowing occurs faster where no UV light is present, so in door shuts or even under ornaments on window sills are both prime candidates.
New builds are usually prime candidates for yellowing woodwork. Often the doors have been sprayed in a factory with water based so will remain white, in contrast to the door frames which will be yellow.
Why do contractors use oil based? . Contractors tend to go with what they know. So they have the skill set to apply oil-based, and they don't like to change. It's easier to get a decent finish as it dries slower and therefore levels slower. Also a standard oil gloss is cheaper than modern alternatives.
I personally prefer to offer a water based option, it is quicker to apply, many of the products don't require undercoat if the surface is prepared correctly and some will cover properly with two coats.
Do you still like a high gloss finish? Some people do but I have found it is a little out of fashion nowadays. I tend to recommend a satin, eggshell or semi gloss finish. The reason for this is that water based cannot achieve the same level of sheen as the traditional oil based yellowing paints, however this is improving all the time.
So next time you get a decorator check his specification for your interior woodwork!