Uranus and Neptune are both considered as gas giants and have a very similar atmosphere.
Neptune's atmosphere is composed of 80% molecular hydrogen, 19% helium, 1.5% methane, 192 ppm hydrogen deuteride and 1.5ppm ehtane. The atmosphere of the planet also contains ammonia ice, water ice, ammonia hydrosulfide and methane ice.
On the other hand, the atmosphere of Uranus has 82.5% molecular hydrogen, 15.2% helium and 2.3% methane.
Methane is what gives both planet their bluish appearance.