If all your color/brightness/contrast settings are adjusted, there's not much more you can do. It would be time to look for a new one.
If you have the required skills, you might examine the power supply board inside the monitor for a failed (usually bulging) capacitor.
These are readily available at an electronics hobby retailer. Replacement is simple if you note the correct values from the existing capacitor and observe the correct polarity when soldering the new component in.
If you don't understand the above -- do not attempt repair but hand the monitor over to a skilled technician.