Im not sure if this is necessarily the right place for this but i figured someone might have had a similar issue. I have a Yamaha RGX A2 in white but over time its turned yellow. I assume its the clearcoat rather than the actual colour underneath. Is there a way to reverse this?
Everyone has a right to their own preferences, but the music store that I worked at just loved to see those older guitars with a yellowed finish come into the store in trade for the shiny new guitars. Many customers would choose the older, yellowed, heavily played instruments over the shiny new guitars sometimes at higher prices. There seems to be a certain mystique about aged instruments, but it seems they need to show their age to be desirable. Go figure.
I am not a chemist, but I believe this can be an effect from sun exposure. Something about UV rays can cause it to turn yellow. Possibly by Lessing the whitening agents in the paint (titanium dioxide?). However, polyurethane also yellows over time, which, like you, I'm guessing is the culprit. It is possible to sand off the outer layer, and recoat it, but like others have said: it might not work, and the yellowing is cool.