I finally took some spectra of Uranus in early December and this allows a good comparison of how the two planets can be seen in their differences, especially in colors.
The first graph is a comparison of their respective albedo, i.e. their capacity to scatter back through space the amount of sunlight they receive. These curves are the spectra of the planets whith the Sun spectrum being removed.
The second graph is a comparison of their respective color spectrum, i.e. their light flux as they come to us, before being filtered by our atmosphere, instrumentation, camera or eye. These second curves are the solar spectrum multiplied by the albedos of the planets. They have been re-scaled so they cross at 440 nm, where their respective albedo in blue light is equal.
Both graphs shows that while the relative brightness of the two planets can be considered as equal in blue light, despite a different slope (I find the albedo in the Johnson B photometric band to be equal for both at 0,55), the color differs more noticeably in green, red and infrared where Uranus is slightly, but definitely, more reflective than Neptune.
Uranus is going to be perceived at the eyepiece as less blue than Neptune, slightly more green, and more and more colorless to a point when using big telescopes and/or more clear skies, as the eye is turning into its photopic mode of perception (=bright light) because it then becomes more sensitive to the red emission of the planet. Digital images should be processed accordingly!
It will be interesting to see in the coming years or decades, as the bright polar region of Uranus is slowly turning towards us, if the difference increases again. Uranus is going to be white !
Edited by CPellier, 13 January 2020 - 04:52 AM.