If what you see on your Mac isn't in color anymore, or in other words everything's in shades of gray like a black and white photo, probably the display has been switched to grayscale. To change it back, open
System Preferences >
Universal Access >
Seeing and uncheck
Use Grayscale.
Written by jchuzi with contributions from cyn, dianne and Hal Itosis in Changing display from grayscale.
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