I think this happens commonly, it'* the effect of the sensors in your eyes still reacting to light you've already seen. The same reason that you see spots after a camera or the optical illusion of looking at a red/blue picture of the house on the hill, then looking at the black and white picture, which turns into color because of the residual color you're still seeing. I'll look it up and see if I can find a better explanation...
Edit: I haven't found any really useful info, but it should also help if I mentioned that your rods and cones have sensitivity changes, as in once they fire off, they need to "recharge" and that they adjust to light and dark stimulus. I'm not sure if I'm right, I just learned a bit about eyes in my Psychology course.
So basically what I'm guessing is that you're getting a residual amount of sensory input from what you've already seen. A good way to test this would be to close your eyes and see if the patterns of colors correspond to the light stimulus around you. IE, if you've been looking at a computer monitor, there would be a rectangle in the center of your field of vision, probably blue or purple. Also, if you're in bed in the dark, and you close your eyes, the amount of color you see would be minimal. Just my $.02.
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