+1
If you have a picture but in black and white with a RGB mod, it is without hesitation a saturation issue.
The wiring of your mod is OK, dont mess with it.
The saturation pot is maybe not connected or misajusted.
If you want to display all the spectrum color of your console, use the 240p test.
it will not be as good as you think (or want) but it is the best way to know if all colors of your pcengine are properly displayed.
good luck.