No, it's not the ROM, you have a busted ECS Denise, or the related color chip is the problem (or even a faulty SMD resistor).
If you have the schematics (and the skills), trace the green signal along the way from Denise to the CXA1145 (video encoder), U31 (74HCT244), Q212 (green transistor)+ associated resistors, and other areas involved.