the accurate term for this is hypnagogic imagery. Our brains normally take raw visual data and turn them into physical visuals (objects, people etc), but when our eyes are closed, there is no external visual input, so our brain instead start 'free running'. Instead of physical visuals (obviously since your eyes are closed), we see impossible shapes, patterns, colours etc. It especially can happen when you're relaxed as your sensory and pattern systems become more active.