It’s an HP M90 (I assume that’s its name, that’s all there is written on it). About a week ago it began doing something odd: it’s randomly started to turn green–that is, the picture goes from being normal color to just shades of green. How long it stays like this varies, sometimes being just a flicker, other times lasting for several minutes.
I know from past experience that a loose cable can cause color distortions, but that doesn’t seem to be the problem here. I disconnected it, cleaned it, and reconnected it tightly, but that didn’t change anything at all.
There’s no pattern to it that I can see. Sometimes it does it when it’s first turned on, but other times not. Sometimes not five minutes elapse between “greenings”, other times it’s fine for several hours.
If you have another monitor, try connecting it to your computer to see if it’s the video card. Alternately, connect this monitor to another computer to see if the same symptoms appear.
Typically in this case, something is wrong with the monitor cable. The cable sends red, green and blue (RGB) signals to the monitor. When something is wrong with the cable, some of the signals are not being sent (in your case, red and blue), or are not “strong enough” and thus you get an abundance of another color (in your case, green). If you play with the monitor cable (jiggle it up and down and\or side-to-side) you might see the screen return to normal. If this is the case, it is almost certainly the cable. If not, then the cables either totally screwed or the monitor is in fact dying.
Unfortunately, there’s no easy way to fix just the cable. I’m using an SGI monitor that has a replaceable cable, but most VGA cables these days aren’t changible. If the end of the cable on the back of your monitor looks exactly like then end that attaches to yoru computer, go to Radio Shack and buy a new VGA cable for around $15. If the cable doesn’t unscrew, you’re screwed. See what’s on sale at Best Buy or Office Depot this week! You can repair monitors, but it typically takes too long and isn’t very cost-effective. Also, do what Mr. Blue Sky suggests and see if you can isolate the problem first. 99% of the time it’s the monitor that’s failing, but you could always be that 1%, ya know?
I’ve had this problem on many monitors since the 1970s. As others have noted, it can be a cable (which may just need reseating and tightening, not replacement) or video card problem (which may be helped, oddly enough, by changing the driver, reseating the card, or changing the refresh rate)
It can also be something wrong in your monitor, but it may be years from dying. Often you can fix it by decreasing your refresh rate slightly or cleaning the inside. (warning: charged capacitors inside the case can store high voltages for many days). Loose connections and other trivial problems can also ause this. I’ve had the problem disappear after I opened a monitor up and poked around (I used to fix TVs as a teenager) I jokingly diagnosed it with “Attention Deficit Disorder” – the monitor just needed some attention.
But yes, it can be something more serious, and progress until your monitor dies. I just thought I’d emphasize the less serious causes. Individually, each cause may be uncommon., but I’ve resuscitated many friend’s monitors with simple measures, and I’ve seen monitors suddenly work fine once they got to my bench - probably just a matter of vibration during transport.
Just last month, I put an old 15" monitor back in full time service for my sister. It’s been in regular or intermittent use (as a spare, or a temporary server monitor) since it developed the exact problem you describe in 1995 (the first time). The problem has always been fixed by the measures I listed above. Though you may be tempted to simply replace your monitor, it may well have a lot of service left in it! If all else fails consider finding a TV repairman who knows how to do something other than replace CRTs or motherboards (they’re getting rare) to poke around and clean it out. It may not cost more than the shop’s minimum fee (often $35 or so)
You can repair monitors, but it typically takes too long and isn’t very cost-effective. Also, do what Mr. Blue Sky suggests and see if you can isolate the problem first. 99% of the time it’s the monitor that’s failing, but you could always be that 1%, ya know?