I also found these a few months ago and am using one in the eBook i'm writing for digital artists. Generally, our visual systems work remarkably well, but if you present just the right unusual stimulae things go way wrong. The fact that you have to build these very special visual experiences, called optical illusions, in order to experience the problems shows how well our vision usually works and how we have learned to compensate for the inevitable limitations. I think there is a lot to be learned from optical illusions - and they are a lot of fun too. That 3rd one is really amazing, must be new, since i didn't see it back then. Probably only works with yellow, which holds a special place in the spectrum, in that there are very few sources of actual yellow wavelength in nature. Most often when we experience yellow, it is from equal and strong stimulation of both red and green receptors at the same time. Pure yellow is such a narrow energy band that it usually doesn't have enough amplitude (brightness) to be seen as anything more than dark grey.