Color vision requires the cone system, which requires daytime illumination. When your eyes are closed, only the rod system is working. Rods are used for dim illumination, side vision and motion detection. This is why you don't see color and forms when your eyes are closed.
Visualization exercises will not change the physiology of your eyes....
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If that is true then how do other people I know close their eys and see vivid, bright detailed colors and pictures?
That is an excellent question....and, I must admit, I don't know.
It doesn't make physiologic sense that these people you know can see vivid detailed colors and pictures with their eyes closed....... but then it is a known fact that some individuals can do things that are not easily explained. I am very slow to poo poo things I don't understand -- if you do, you miss a chance to learn and grow.