To tone down or To tone up

(a.) To cause to give a lower tone or sound; to give a lower tone to; hence, to moderate or relax; to diminish or weaken the striking characteristics of; to soften. “The best method for the purpose in hand was to employ some one of a character and position suited to get possession of their confidence, and then use it to tone down their religious strictures.” Palfrey.

(b.) (Paint.) To bring the colors of into harmonious relations as to light and shade;—said of a picture. “Its thousand hues toned down harmoniously.” C. Kingsley.

— To tone up, to cause to give a higher tone or sound; to give a higher tone to; to make more intense; to heighten; to strengthen.