I don't have my schematics for the bak board (small top board that has the PROMs on it) available at the moment, but IIRC, someone has posted somewhat recently which chip does which colors on which monitor. I think there's a little bit of circuitry as well... I think a transistor or two on the output of each color, so it's possibly not the color PROM.
You can pull one color PROM at a time to determine which is which, and once you figure out which one is top monitor blue, you can swap with one of the other PROMs and see if you get some blue and lose the other color, or are always missing blue.
DogP