yep light deprivation will turn brilliant white gloss into a shade of magnolia in a couple of weeks.
An example.. I had to go back 2 weeks later to a room where I had painted the trims/ frames brilliant white. They had a new door fitted and wanted it painting. The door after I had finished was brilliant white, the 2 week old paintwork was quite yellowish. I had to drape a sheet over the door to starve the light. it only took 2 days for the door to yello down to the 2 week old paint..
The fact they had the door fitted after I had decorated I have not sussed out just yet...
Mostly happens with solvent based paints, all brands.. consider water based products= yuk..... or when glossing try a colour something like old 'english white'?..