Original: As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's color.
Simplified: Neptune's blue color is much brighter compared to Uranus', which has a similar amount of methane, so there might be another reason why Neptune is blue.