Check the cables.
Color changes are almost always a faulty wire in the cable. Try working the cable back and forth near both connectors. If this causes the color change, you could have a broken wire in the cable or a loose pin in one of the connectors.
Check for speakers, florescent lights, fans, cell phones, radios, and any other electrical device that may cause a wide range of monitor problems. Pluggins a radio or fan into the same receptacle can also cause screen flickers. Some monitors may cause other monitors to degauss when they are turned on.
Check the refresh rate. Upgrading from 95 or w.x to 98 or NT+? The refresh rate may be improperly calibrated. Be careful - you could harm your monitor. Excessively high refresh rates can damage video displays. Check the manual for the maximum refresh rate. 70-85Hz is common.
Check the video card. This is rare. Make sure it is securely seated on the motherboard.
Check the monitor itself. The flicker is probably due to dry solder joints on the CRT base (a small PCB on the end of the CRT). It should be a fairly simple task to re-solder these. Caution - the CRT is essentially a huge capacitor with a charge of up to 25,000 volts!!! If you have no experience with electronics, don't take the back case off - send it away! A TV repairman should be able to fix it.
If the above do not fix your issue, it may be time to donate your old monitor to the neighborhood computer geek or a non-profit near you. We do pick up used computer equipment, repair what we can and then deploy what is working to area non-profit organizations.