According to Steve Bell of the UK who used to post (perhaps still does) in the sci.electronics.repair and comp.sys.mac.* forums of Usenet, the Apple 17" multiscan monitor has three largish surface mount resistors on the circuit board which hangs off the CRT yoke which are not soldered properly. When a color goes out, the first thing to check is whether one of those resistors needs resoldering/replacing. I think they are 330 Ohms, but my memory is leaky.

You may also wish to do a deja-news (Advanced Google Groups) search in the comp.sys.mac.* hierarchy for postings about that monitor by Steve Bell.