Being unlit is odd to imagine, but I am glad it works for you. So when humans are not awake, they're all just - empty? Not doing anything?
Rithel are each awake half the time, for one swap, but we only rest our bodies for a little bit of time. Someone's color is the color of the light that makes them alive. Yellow light all ultimately comes from the sun when it is yellow, and teal light comes from the sun when it is teal. The only real differences between the colors are the devices that can be made with the light, and when a person is awake. Yellow people also tend to be more outward-focused than teal people, but I am not sure if that is because of the light or something else.
I'm not sure what you mean by describing light as frequencies. Usually when I see light, it is constant, or lasts for a little bit and then stops.
We also use light to see, although we mostly use fire and plants that reach down from the surface, or glowing cave slime. We've read about people who live on the surface and spend a long time in bright light developing sore skin, so I suppose that this must just be a difference between our species. If a rithel cannot get light, either from the sun or from food, they die the next day. That is what happened in the case of the infant Chayis described. I agree that such an experiment would not be a good idea.