I know how the eye can get an image "burned in" if it doesn't change for a long time, and the burned-in image is actually a negative of what the eye was seeing. The retina cells (or maybe their processors in the brain) will always try to neutralize a scene, as a way to eliminate noise inherent to the eye (such as blood vessels through the retina). The only thing that keeps us from going blind is the fact that our eyes are always moving and the scene is always changing. If you were to look at a static scene non-stop and your eye could not move, the scene would eventually disappear. But this is difficult to try because the eye muscles have a micro-jitter built in that we can't consciously stop.
Well anyway, this is all related to what I actually want to talk about, which is the burn-in effect itself. The fact that the burned-in image is negative results from the goal, which is to bring all the color and brightness values together into one. That means anything that is red needs to be tweaked towards the cyan in order to appear "fixed". So if you look at something red for a long time, eventually it won't seem as red. But if you then look away onto a solid white object, you will see a cyan version of the red object for a short while.
This is also why when you come indoors from a bright sunny day, everything looks green for a little while... because green is the opposite of the sun's light.
Ok, so we've established the negative image effect...
So then why, when I look at a deep purple object (405nm - nearly UV), is the "negative" image also purple?
Things that make you go "hmmm..."