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.

Here is the top half of Figure 1 from the Irwin et al. paper:

"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). There are a lot of details in this new model, and 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 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 (darkening the red part), and more haze particles in one of Uranus' layers (brightening the blue part).

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.

Here is the top half of Figure 1 from the Irwin et al. paper:

"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).

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.

Here is the top half of Figure 1 from the Irwin et al. paper:

"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). There are a lot of details in this new model, and 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 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 (darkening the red part), and more haze particles in one of Uranus' layers (brightening the blue part).

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.

Here is the top half of Figure 1 from the Irwin et al. paper:

"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). There are a lot of details in this new model, and 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 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 (darkening the red part), and more haze particles in one of Uranus' layers (brightening the blue part).