If Neptune and Uranus have similar composition then why is Neptune a darker blue than Uranus?
This is an intriguing question, with a solution suggested by the Irwin et al. (2022; arXiv:2201.04516) paper that inspired the YouTube, NewScientist, ScienceAlert, Phys.org, etc. coverage of the past couple days. Still, I would wait a few years to see if others have alternate ideas before calling this case closed. Also, the paper needs to go through peer review.
"Human eye" curves are compared with Hubble Space Telescope spectra of the two planets. A couple things to note: Neptune is slightly more reflective where human blue optical cones peak, while Uranus is slightly more reflective where the red cones peak. So this is the color difference that the OP was asking and answering about. These Hubble observations of Uranus and Neptune were already published in 2009 and 2011 based on 2002 and 2003 observations.
What is new from the Irwin et al. study is a model, or theoretical description, of the haze and cloud layers present in both Uranus and Neptune. The paper actually argues for a simple structure of 3 haze/cloud layers on Uranus (plus one additional layer on Neptune), mostly similar between the two planets. Earlier papers assumed all the layers were either hazes (like photochemical smog) or ices (of methane or hydrogen sulfide), but this new model has hazes mixed into all the layers (except for that extra methane-cirrus layer on Neptune).
The "Aerosol-2" layer is the one claimed to account for color differences. In this new model, the Aerosol-2 layer is a little bit absorbing, especially at UV and blue wavelengths, which is a pretty standard assumption for haze layers. But it is located deeper in the atmosphere, where previous studies had called for a methane-ice condensation layer. Other scientists may use different haze/cloud models to describe Uranus and Neptune data. So it would be a good idea to wait and see whether the new Irwin et al. model gets challenged by others. Also it would be great to actually send probes to these planets and see whether any of these models are correct.
In the new Irwin et al. model, the main factors affecting the visible color of the two planets---the differences between the pink and black lines above---are more gaseous methane on Neptune (making Neptune less reflective than Uranus at red wavelengths), and more haze particles in one of Uranus' layers (making Uranus less reflective than Neptune at blue wavelengths).
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
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.
- Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots by Patrick G.J. Irwin et.al (arXiv:2201.04516)
- Why is Neptune's Blue Hue Different from Uranus?
According to https://phys.org/news/2022-02-explanation-difference-blue-hues-uranus.html it's because Uranus has a thicker atmosphere than Neptune.