Vision research over the past 30 years has gradually proven that forbidden colors — reddish green and yellowish blue — are real, though some scientists still don't believe it.

The sky looks blue on a sunny day. But at night, ... It is a glow-in-the-dark night sky too, colored a mix of green, yellow, and red. Its colors result from scattered sunlight, oxygen, ...

They can be red, yellow, blue and white, but not green. It all has to do with the way stars produce light, which is very different from other astronomical objects. Comets, ...