It is difficult to believe this illusion even after learning the secret. The brown tile in the center of the top face, and the brown tile in the center of the front face of this rubix-like cube are exactly the same shade of brown.
If you are in doubt move the slider-control to the right to fade the background and reveal a color key. As the background fades to black you can see that the two tiles are identical.
Move the slider all the way to the right-hand stop to view the color key and the background at the same time. Notice that the color key appears dark brown at the top and much lighter near the bottom.
The power of contrast causes this illusion. One of the tiles is surrounded by light things, and the other is surrounded by dark things. Contrast can be heap-big powerful mo-jo.