Yellow apparently means that the CPU hit an error before the exception handling code (the bit that brings up a guru meditation message) was loaded.
Cause could unfortunately be any number of things, My A4000 produced them for bad memory and a bad power supply, and my A2000 because the wind was blowing in the wrong direction or something
but it could also be due to a defective zorro card, or a poor connection to the RAM/CPU, or some of the other ICs, or potentially board damage from a leaky battery.
My first action would be to reseat everything that can be reseated, and make sure the edge connectors on your zorro cards, and their respective slots are clean (same goes for the daughter board) slots can be cleaned by covering the edge of a piece of thick card with alcohol or contact cleaner and pushing it in and out of the slot a few times.