Multicoloured, but it can possibly look fine in brown (7)
I believe the answer is:
rainbow
But I can't explain it at all! Could you help me understand?
(Other definitions for rainbow that I've seen before include "Arc caused by refraction of the sun's rays (7" , "Multicoloured atmospheric phenomenon" , "Heavenly phenomenon" , "Refracted light in sky" , "'The - - ' by D H Lawrence novel" .)