Since you have a Mac Mini, you must have an external display - could it be that this is defect?
Perhaps it's the cable - e.g. if you have a VGA connection, it could be that one wire is broken, or one pin of the connectors has bad or no contact. Unplug and plug back in both ends to see if it goes back to normal.
If you have a greater defect but no other ways to check, you can only verify by testing with another screen/computer/cable.
If all hardware seems okay, maybe for some reason the color profile on the Mac has (been) changed. System Preferences -> Displays -> Color tab, check which profile is active. Try one of the generic (RGB) profiles.