You described the colors very well, and that leads me to believe that your display preferences in the System Preferences Control Panel may simply have switched to automatic. It changes the hues to what they think are easier on the eyes for day and night. Go to
“System Preferences” (it’s also in the Dock by default)
and adjust it the way you like. BTW, if it seems to be happening at times other than Sunrise and Sunset, make sure the Time Zone is set for your local area and proper Daylight Savings (if applicable) or Standard Time.