If I zoom into the point where I can clearly see the grid pixels, the grayscale areas in between the grid lines still have an illusion of color for me. I think the examples images have too fine of a grid that makes the illusion seem unimpressive and trivial. But if you open them in an image editor (or just use your browser's magnify feature) and zoom in, there's a point where things change from being just a blurry color image to a surprising illusion.

Hm, not sure. We can easily resolve the structural detail within each grid, so does our brain just interpolate for colour? I wonder would a green grid on the sky make us perceive a green sky, or do we interpolate it here because we know the sky should be blue.