Comment by hatthew

I would call it an illusion because if you pay attention you can clearly see that the color you perceive isn't actually present. You can see white on an RBG computer screen since your eyes simply don't have the resolution to discern the subpixel colors. However, in a dithered image with only black and white, you perceive gray, but you can also tell what the reality is without much effort. Personally, I think that fits the definition of an illusion.