Abarim Publications' online Biblical Greek Dictionary
The Greek adjective λευκος (leukos) primarily means to be white, and secondarily to shine or glitter. In the Bible it's used for clothing ("white as light," Matthew 17:2; Acts 1:10), hair (Matthew 5:36, Revelation 1:14, "white like wool, like snow"), of a stone (Revelation 2:17), of a horse (Revelation 6:2), of a cloud (Revelation 14:4), and of God's throne (Revelation 20:11). White is also the color of a crop ready to be harvested (John 4:35).
From this adjective comes the verb λευκαινω (leukaino), meaning to whiten or make white. It's used twice in the New Testament: Mark 9:3, "his garments became radiant and exceedingly white" and Revelation 7:14, "garments made white in the blood of the Lamb."
In texts other than the Bible, the root of these words shows up in an enormous array of words, from having white blossoms to having white arms or to be with white horses. The noun λυξνος (luchnos) denotes a portable lamp, and the very fair word αμφιλυκη (amphiluke) denotes the morning twilight. In our times this Greek root survives in the word leukemia.
Our root also exists in Latin, where it appears to predominantly mean to shine and only secondarily to be white. The verb luceo, means to be light, be clear, shine, beam, glow, glitter (say Charlton T. Lewis and Charles Short, A Latin Dictionary). Other words that contain this verb are luce, meaning by day(-light); lucerna, meaning oil lamp. And words that are related to the verb lucere are the familiar nouns lux, meaning light or day(-time), and lumen, meaning light or brightness, which survive in English in words like luminous and illuminate, and even luna, meaning moon.
Note that in Hebrew these words are covered by the root לבן (laben), meaning to be or become white (see the name Laban), except that לבן (laben) never means to be light. This is really quite clever since there's nothing inherently light-related to whiteness. A white surface will even reject light (because of which it looks white), but a black surface will absorb light and is as such much closer related to it.