Interesting. I see coloured "lights" behind my eyes most of the time when I close them, most noticeably when I'm tired and in the dark. I see them in front of my eyes too if they're particularly strong and it's dark. The effect is different than when I press my eyes and it's different to the "sunspots" you get if you stare at a light souce too long.

The colours I see are always an unpleasant luminous yellow-green and a rather pleasant shade of lilac. They're always swirling and changing, swapping in and out with each other.

I don't know why this happens, or if it's related to anything described above, but I suspect it's probably due to eye strain, since it's most pronounced at the end of a hard day.