Hi. I recently got myself a neo-geo one slot (chuffed to bits with it ^_^) and I have noticed that the full bright colours (full red, full blue, full green, full white, etc) loose a little of their sharpness (get the same on my Mega Drive, my master system, and my other game pcbs). Now knowing my luck simply putting a variable resistor on each colour line isn't going to work (go on, prove me wrong!).
How would you reduce the RGB output?
Actually, you were spot on. Dropping the R, G, and B lines across 75ohms usually does the trick.