I know, I know, this is undoubtedly a widely known optical illusion, but I had to create and share my own version after stumbling across it in the wild. Watch the animation cycle through all four frames, 2.5 seconds per frame. Notice anything different about the spectrum when it flips from green as opposed to when it flips from gray?
The spectrum and greenish frames image are derived from Project Octant Part 13: Bug Hunt, which is where I saw this effect thanks to the Page Up key and the browser window being just the right height. I hope Shamus won’t be too irked that I swiped copies to repurpose here.