I bought one the assembled video mod boards from LittleJoe and installed it last night. Photos of the Combat atrract screen are in the spoiler.
As the colors cycle, the picture comes and goes and switches between the states pictured. The best output is when using the in-game colors: I can see the scores, background, and left tank clearly in correct colors but the picture is shifted down vertically several lines and the rows all oscillate towards the right, shearing the picture. My impression is that the variation is due some of the colors going out of range, and the shearing means something's wrong with the sync, but I can't see any obvious problems with the wiring.