Both Neptune and Uranus contain large amounts of methane gas in their atmospheres. Neptune does contain a large amounts of hydrogen and helium but still has a significant amount of methane. This gives Neptune a blue hue while Uranus is a green blue color. The reasoning behind this is similar to looking up and seeing a blue sky here on earth. Light bouncing off molecules here on earth allow us to perceive that the sky is blue. When the light from the sun hits Uranus' and Neptune's methane rich atmosphere the methane scatters the light making it appear blue.
