The reason why this difference could not be explained until now is because of the absence of similar data for comparison. As per the previous studies of the spectrum, each planet concentrated on individual wavelength regions, while researchers developed a new comparison with a single atmospheric model that matches observations of both planets from the NASA/ESA Hubble Space Telescope, as well as the Gemini North telescope and the NASA Infrared Telescope Facility. According to the model the haze around Uranus is thick around Neptune and the researchers have reported the same in the Journal of Geophysical Research: Planets.
According to a release on the ESA Hubble website, Uranus has a sluggish atmosphere which makes it lighter than Neptune and in case there is no haze in Neptune and Uranus atmosphere, both planets project a similar blue colour that results in blue light getting scattered in their atmosphere.
As per the model that leads to this conclusion there are three aerosol layers in Neptune and Uranus atmosphere and the key layer that affects the colours is the middle layer, which is the haze particles layer which is thicker on Uranus compared to Neptune.
According to the team, on both planets methane ice condenses onto the particles in this layer that pulls the particles dee in the atmosphere and since Neptune has a more active turbulent atmosphere than Uranus, the atmosphere on Neptune is more efficient in churning methane particles into the layers of haze. As the layer of haze is thinner on Neptune compared to Uranus, this results in Neptune’s stronger blue shade.