I have an blue imac, G3, running OS 9.2 and only recently at startup it displays a green/gray screen, maybe striped, before actually booting up. Also, when I tell it to shut down, instead of just turning off nicely, it goes back to that green screen, then the green gets smaller and more bright and moving into the center of the screen until it's just a green blip in the center until it disappears. Otherwise it seems to work normally. Any ideas? Thanks.

Sounds like your analogue board is dying, although it might just be a couple of cold solder joints. Unless you're the type who's skilled in repairing that sort of thing, the repair will probably cost about as much as the iMac is worth. p

thanks for the response. in your opinion and experience, is this the sort of thing that will worsen to the point where i'll either have to fix or replace it? or will it just remain an annoyance/oddity but not really affect performance? also, is this common in imacs?

Analogue boards don't fix themselves, and they don't heal themselves either. If it still works OK otherwise, I'd leave it alone until it gets to the point where it won't function normally any more. This is probably more common than it ought to be in iMacs due to the design of the computers and the way they remove heat from the case. p