When it comes to what happens during meditation, remembering that we are dealing with the unconscious, anythings fair game. For some people their internal reflection brings up intense visions. For others, feelings and sensations. Many, however, see colors when they close their eyes and take the journey within.
Deepak Chopra, in this Q & A Intent post, discusses what it means to see colors during meditation. According to Chopra, while colors might be pretty to look at, they are merely the result of the refocusing of perception. Important, is to remember that the purpose in practicing meditation is to bring the mind back to the meditation itself and not to be taken by distractions.
The colors you see are the result of the refinement of perception that occurs during the meditation. But that subtle perception is not the ultimate goal of the meditation. So rather than letting your attention get distracted by the passing scenery, it is better to gently bring your mind back from the colors to the meditation itself. That way you keep your flow of awareness inward and directed toward your growth. Now when you meditate it would be better to move away from the mirror and sit comfortably in natural daylight, so as not to encourage the expectation of looking for colors.
Read more from Chopra on seeing colors during meditation here.