Authors:

Keep it simple: In general, interfaces should use simple geometric forms, minimal contours, and a restricted color palette comprised primarily of less-saturated or neutral colors balanced with a few high contrast accent colors that emphasize important information. Typography should not vary widely in an interface.

Alan Cooper, Robert Reimann, David Cronin (2012). “About Face 3: The Essentials of Interaction Design”, p.308, John Wiley & Sons
Keep it simple: In general, interfaces should use simple geometric forms, minimal contours, and a restricted color palette comprised primarily of less-saturated or neutral colors balanced with a few high contrast accent