[expat2]

I suspect that it is an optical illusion, like the one you get if you stare at a white spot for a little while, then look away, and see a negative black spot. The last bit of the sun is red at sunset and when that disappears, you can sometimes see a color-negative green flash.

[Kirkwood]

It is not an afterimage illusion. It is refraction.

[Oztrich Boy]

If it's an in eye optical illusion, how can it be photographed?