As a student, concerning my paintings, I often heard, ...muddy palette, darks and lights are indistinguishablePaint the whites whiter and the blacks blacker.  Teaching color Theory I passed down similar suggestions to students.  I would say, "Warm up the blacks.  Cool off the whites."  I heard the words spoken from my mouth but they skewed tangentially from their intent.  Words out of context can sound silly if not incredibly powerful.