It's not actually the atmosphere itself that makes Uranus and Neptune look green, but rather the way their atmospheres absorb and scatter sunlight. Here's the breakdown:

* Methane: Both planets have atmospheres dominated by methane (CH4). Methane absorbs red light very efficiently, leaving behind the blue and green wavelengths to be scattered back into space.

* Scattering: The blue and green light is further scattered by the particles in the atmosphere, a process called Rayleigh scattering (the same reason the sky appears blue).

* The result: The combination of methane absorption and Rayleigh scattering gives Uranus and Neptune their distinct bluish-green color.

Important Note: Uranus appears slightly lighter blue compared to Neptune. This is likely due to Neptune having slightly more methane in its atmosphere.

While the green hue is dominant, it's worth noting that these planets also have traces of other gases, and their atmospheres are incredibly complex. The colors we see are just one piece of the puzzle when it comes to understanding the unique properties of these ice giants.