1650s, originally of paintings, "the brightest part of a subject," from high (adj.) + light (n.). High lights came also to mean the lighter and brighter paints and colors used in making pictures (as opposed to middle tints and shade tints), and the terminology carried over into photography and engraving. The figurative sense of "outstanding feature or characteristic" is by 1855 (as highlights give effect to a picture) but was not common before c. 1920. Hairdressing sense is 1941. Related: Highlights.
1861, "to give high lights to" (a painting, engraving, etc.), from highlight (n.). Figurative sense of "give prominence to, emphasize" is by 1944. Hairdressing sense is 1942. Related: Highlighted; highlighting.