It sounds like one of you electron guns in your tube has gone - much as you already suspect. This is the most likely cause, but it could also be due to some part of the electronics that controls the green gun has stopped working. If the gun has gone then you'll definitely need a new monitor as they are not worth fixing because of the cost, and good monitors are very cheap right now. If the problem is with the gun, then it will be permanently off and not intermittent. If you experience any occassional return to the correct colour then you may be able to fix it as it is most likely due to a poor contact somewhere in the green gun circuit.