Logically, a color, green etc., is a 'simple' notion and cannot be explained terms of anything simpler. With color we have to revert to a different description,

Logically, a color, green etc., is a 'simple' notion and cannot be explained terms of anything simpler. With color we have to revert to a different description, here wavelength. But wavelength is not human perception (and we can't explain such perception in simpler terms).