Neptune has a distinctly bluer colour whereas Uranus is a pale shade of cyan. The two farthest planets from the Sun are similar in mass, size and atmospheric composition but Neptune is distinctly bluer than its neighbour.

Scientist's Explanation of Neptune and Uranus 

• A hazy layer that exists on both planets is behind the different shades of blue. Their appearances would be identical if it wasn't for this haze.
• Excess haze on Uranus builds up in the planet's stagnant, sluggish atmosphere, which makes it appear a lighter tone than Neptune.
• Using observations from the Hubble Space Telescope, the NASA infrared telescope facility and the Gemini North Telescope researchers developed a model to describe aerosol layers in the atmosphere of both planets.
• The researchers explained that the model involves three haze layers at different heights. On Uranus, the middle layer of haze is thicker than on Neptune, which affects the visible colour.
• On Uranus, methane ice condenses on the particles in the middle layer of haze, which forms a shower of methane snow that pulls haze particles deeper into the atmosphere.
• Neptune's atmosphere, on the other hand, is more active and turbulent than that of Uranus, suggesting that it is more efficient at producing the snow which removes more of the haze and keeps Neptune's middle layer thinner.