While ST is correct when it comes to some all-in-one Macs, I have yet to see this happen with an iMac, let alone one of the older all-in-ones that did color like the older 68K Macs or the Power Mac 52xx/53xx/54xx series. As for the G3 All-in-One, I haven't ever used that computer so I couldn't say one way or the other.
Mind you, I HAVE had an Apple MultiScan 15" monitor lose a couple of it's color guns and only display green, but there was a known defect with the monitor.
As mentioned before, it sounds like it either needs a firmware upgrade or the monitor preferences for the color calibration aren't set properly. Hopefully it's nothing beyond that.