A part of screen or entire screen color of monitor changed to red or green or blue or yellow or any other combination of colors

This usually happens when there is problem with the signal cable which connects from monitor to CPU.

Check the end of the signal cable connector for any bend pins which connects to the CPU. If so, slowly, gently and carefully straighten the pins back to straight. Make sure you do not break any pins.

If all the pins of the signal cable connector are perfect and straight, then try wiggling or twisting the end of the signal cable connector slowly to clockwise and anticlockwise and check for the monitor color. If you see there is variation in the color changes then there is problem with the signal cable connector.

Strong advice is to take the monitor to computer hardware technician or any electronics technician and ask him to change only the signal cable connector. The connector cost is very less.

Monitors that come from manufacturers have signal cable connector that is machine/hard molded. Repairing yourself is easy only if you are perfect and have very good knowledge in electronics and soldering because signal cable has couple of wires that has to be soldered to connector with correct combination.