A frame of slowly changing colour.
The change is slight enough that it is perceptible, yet is slow enough that it is hard to see the change as it occurs. The change is perceptible primarily when the eye becomes saturated with colour; the eye detects the change but the retina is overstimulated, so the colour one sees is a combination of a complementary after-image and the actual colour.
This struggle is a physiological response to visual overstimulation; they eye's response is to try to move the overstimulated area to a fresh area of the retina, which creates frames of the previous colour around the current hue.
For best results, give it some time and let the eyes lose focus.