I won't guarantee thats what caused the previous set to go yellow, but I've read that on the forum multiple times; the coating on new tyres reacts with the paint and basically the walls rapidly deteriorate. They recommend around 1000km to wear them in and let the coating piss off, which make the walls last longer.
try cleaning them with a toothbrush & eagle one wheel cleaner, wear gloves though, cus that **** burns your skin. This is what I have done on the white & blue walls to stop them from changing colour.