Tl:DR: According to a team led by planetary physicist Patrick Irwin of the University of Oxford in the UK, an extended layer of haze dilutes the hue of Uranus, resulting in a paler color compared to Neptune

Long answer

Uranus and Neptune has very similar structural and atmospheric composition (A small, rocky core is surrounded by a mantle of water, ammonia, and methane ices and a gaseous atmosphere consisting primarily of hydrogen, helium and methane and a cloud top in upper atmosphere. The atmosphere isn't homogeneous but rather layered). Irwin and colleagues analyzed visible and near-infrared observations of the two planets to generate new models of the atmospheric layers which explained the storms and the dark spots of the planets very well.

In their models, both planets have a layer of photochemical haze. This occurs when ultraviolet radiation from the Sun breaks down aerosol particles in the atmosphere, producing haze particles. They called this layer "Aerosol-2 layer", and on both planets it seems to be a source of the cloud seeds that condense into methane ice at the lower boundary and snows deeper into the atmosphere but on Uranus, this layer seems to be twice as opaque as it is on Neptune and this is why the two planets look different. According to the paper²:

Since these particles are found to be UV-absorbing, this explains Uranus's lower observed UV reflectivity and also explains why Uranus appears to have a paler blue color to the human eye than Neptune, since these particles are found to have a roughly white visible reflectivity spectrum.

The lower opacity of Neptune's Aerosol-2 layer also explains why dark spots … are easier to observe in Neptune's atmosphere than in Uranus's.

Below Aerosol-2, there is another layer called "Aerosol-1 layer" which is a deeper haze layer, where the methane re-evaporates and redeposits the haze particles. These haze particles then condense into sub-micron crystals of hydrogen sulfide. The Aerosol-1 is concentrated at certain places which is considered to be the dark spots of Neptune.

References

1. https://www.sciencealert.com/we-might-finally-know-why-uranus-and-neptune-are-different-colors/amp
2. Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots by Patrick G.J. Irwin et.al (arXiv:2201.04516)

Tl:DR: According to a team led by planetary physicist Patrick Irwin of the University of Oxford in the UK, an extended layer of haze dilutes the hue of Uranus, resulting in a paler color compared to Neptune

Long answer

Uranus and Neptune has very similar structural and atmospheric composition (A small, rocky core is surrounded by a mantle of water, ammonia, and methane ices and a gaseous atmosphere consisting primarily of hydrogen, helium and methane and a cloud top in upper atmosphere. The atmosphere isn't homogeneous but rather layered). Irwin and colleagues analyzed visible and near-infrared observations of the two planets to generate new models of the atmospheric layers which explained the storms and the dark spots of the planets very well.

In their models, both planets have a layer of photochemical haze. This occurs when ultraviolet radiation from the Sun breaks down aerosol particles in the atmosphere, producing haze particles. They called this layer "Aerosol-2 layer", and on both planets it seems to be a source of the cloud seeds that condense into methane ice at the lower boundary and snows deeper into the atmosphere but on Uranus, this layer seems to be twice as opaque as it is on Neptune and this is why the two planets look different. According to the paper²:

Since these particles are found to be UV-absorbing, this explains Uranus's lower observed UV reflectivity and also explains why Uranus appears to have a paler blue color to the human eye than Neptune, since these particles are found to have a roughly white visible reflectivity spectrum.

The lower opacity of Neptune's Aerosol-2 layer also explains why dark spots … are easier to observe in Neptune's atmosphere than in Uranus's.

Below Aerosol-2, there is another layer called "Aerosol-1 layer" which is a deeper haze layer, where the methane re-evaporates and redeposits the haze particles. These haze particles then condense into sub-micron crystals of hydrogen sulfide. The Aerosol-1 is concentrated at certain places which is considered to be the dark spots of Neptune.

Methane concentration also plays a role here. In Uranus, there is 2% methane while in Neptune, there is 1.5% methane in atmosphere and thus more methane absorbs infrared light and reflect blue light.

References

1. https://www.sciencealert.com/we-might-finally-know-why-uranus-and-neptune-are-different-colors/amp
2. Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots by Patrick G.J. Irwin et.al (arXiv:2201.04516)
3. Why is Neptune's Blue Hue Different from Uranus?