check the video plug at the very bottom of the video board (the part that's like 4 feet long of circuit boards) and make sure the red wire isn't frayed off the plug. I want to say they're IDC so that can happen. or if you have another human available try jiggling the plug and see if red pops in and out.
otherwise these tubes are prone to crap electron guns, you could be missing red output but I doubt it. if the monitor has never been serviced it could be something as simple as the red pin of the signal header has cracked solder and the red just isn't getting in. a free tip is to turn up the red cutoff all the way, if the picture tints red then you'll know the tube has some red and that the drive circuitry is working. K4900/5500 has a whole section on the deflection board where the colors can get lost too, but that's a few things to try.
if you intend on keeping this game you will need more tools and a pattern generator like TPG and better understanding of how stuff works to survive