Does this only happen in Workbench?? Because then it sounds like you have somehow altered the colour slider in preferences :-P
I wonder this because the early boot menu looks normal?? Or did you use a different a1200 for that?
Is this problem present using both composite out, rgb and rf output?? If so, then it is probably some resistor that is burnt out of one colour, because it seems like one colour is missing, which will give the picture a green tint. If this only happen on rgb output, then it could maybe be a bent pin in the rgb output port??
Edit: I see you have tried most of what i suggested after reading the whole thread... :-P It sounds to me like it is very likely that the problem is a resistor or a burnt out trace on the motherboard since this also happens using composite out. I recall someone having the same problem with an a600, and then it turned out to be a dead resistor.
