Hi
If the emulated Amiga acts like a real Amiga, the yellow screen should indicate the following:
yellow -- an unexpected processor exception occured during the initialization of the system, i.e. before the Guru was prepared. This color code can, by the way, also occur if the rest routine is entered from outside the supervisior mode (e.g. by a direct jump). On the A3000, this may be an indicator of defective hardware, as asseccing "nonexistent" memory (of faulty hardware) may result in a bus-error exception.
Taken from "The Amiga Guru Book".
I think you should check your WinUAE settings.
Noster