"dull bluish-green, gray," 1670s, from Latin glaucus "bright, sparkling, gleaming," also "bluish-green," from Greek glaukos, a word used in Homer of the sea as "gleaming, silvery" (apparently without a color connotation); used by later writers with a sense of "greenish" (of olive leaves) and "blue, gray" (of eyes). Beekes says it is probably a substratum word from Pre-Greek.
Homer's glauk-opis Athene probably originally was a "bright-eyed," not a "gray-eyed" goddess. Greek for "owl" was glaux, perhaps from its bright, staring eyes, but this, too, might be an unrelated Pre-Greek word. Middle English had glauk "bluish-green, gray" (early 15c.).