The one with a red circle on a brown background had a really interesting effect; before the circle starts to shrink, the background becomes the same colour as the circle, then moving your eyes made a bright red or green circle appear, each on their opposite side.
I got that, as well as a seemingly random set of changes in apparent brightness. The Magenta one seemed to produce patches of bands in my vision as well, think like blown up versions of edge detection kernels.
