Kelvin rating. 6500k is a yellow tint which is good for plant growth. Plants do not grow well under any other kelvin rating other than 65k to 67k. The following colors are good for all coral depending on whether they are deep or shallow species. 10k is a crisp white, 14k is a crisp white/blue hint. 0 to 30 ft ocean depth color. 20k is a crisp blue tint 31 to 60 ft ocean depth color. The actual colors vary depending on manufacture bulbs.