Illusionary

Look at the upper and lower surfaces of the cube. They appear very different; white below and grey above.

Now take a sheet of paper and poke a couple of holes in it.

Place it over the cube so that one hole is over the 'grey' topside and the other hole is over the 'white' underside, and the rest of the picture is hidden.
You can now see that the two sides are actually the same colour.

The science bit. Our visual system evolved so that we don't just see exactly what our eyes detect. The brain interprets the light signals the eyes receive and makes a best guess at what is out there based on its past experience. In this case, from our experience of cubes in the real world, our brain assumes that the top and bottom of the cube are illuminated differently, and that the underside is in shadow.

Thus our brain deduces that the underside is, in reality, a lighter colour than the top side, and this is what we 'see'.