Look at square A, and square B. Believe it or not, these squares are the exact same color. Use the colorpicker in scratch's costume editor if you are really unsure.
In the case of the checker shadow illusion, the shadow cast by the green shape seems to darken a brighter region. This amazing illusion was created by the vision scientist Edward Howard Adelson in 1995 and was designed to show how well our visual system can break down image information.
