Failing that, it might be worthwhile finding out exactly where in the A500 ROM the screen is set to white.
White screen makes me think it's 1.2/1.3, I don't remember the later versions changing to white screen.
It also might not be trying to make the screen white, the cpu could be crashing and store a stack frame in chip reg space or some bus corruption.
Tracing POST issues is hard on Amiga without an eprom programmer.