The Benary Cross was designed by Wilhelm Benary, a German psychologist, and shows the effect of belonging.

 

The triangle on the upper right appears darker than the one on the left, although both have exactly the same colouring. The triangle on the left is visually part of the cross, so the eye compares the colour shading with that of the cross. Result: the grey here appears lighter compared to the darker grey of the cross.
The rich triangle lies outside the cross. The eye compares the shade of grey with the white background so that the triangle appears darker.