Look at the following image.
Look in particular at squares A and B. A appears to be dark grey, B appears to be white or whitish. But in fact, they are the same exact color.
Don’ t believe it? Me neither! Or at least, I didn’t until I went ahead and deleted most everything that is not A or B from this picture. When you do that, you get this:
This is from the Scientific American web site, where there are a number of illusions in a new piece on the subject of Neuroscience of Illusion. Here.