It is difficult to speculate but video problems (when colours are missing) we have seen when repairing Amigas can be due to a defective op-amp or BT101, broken track due to capacitor leakage or blown capacitors near the video port.
What do you actually see on the screen?
If you see a solid colour screen on power on, the Amiga is displaying the possible error- each solid colour screen represents a specific error.
