Which gas gives Neptune and Uranus their blue color?

Methane, which absorbs red light strongly and lets blue through - so the light reflected back from the atmosphere is blue. The composition of the two planets is very similar, and yet Neptune has always appeared a deeper, more vivid blue than Uranus, which was a small mystery for decades.

It turned out to be partly a processing artefact. Voyager 2's images of Neptune in 1989 were contrast-enhanced to bring out cloud features, and those enhanced images became the ones everyone saw; a 2024 study reprocessing the original data concluded the two planets are much closer in colour, both a pale greenish blue, with Neptune only slightly bluer. The iconic deep blue Neptune is, to a significant degree, a picture that was sharpened for scientific reasons and then reproduced for thirty-five years.

The remaining real difference is haze. Uranus appears to have a thicker layer of methane haze that whitens its appearance, and it is also nearly featureless because it radiates almost no internal heat - Neptune radiates over twice what it receives from the Sun and has weather, including the fastest winds in the Solar System, while Uranus is largely still.