The illusion is that the red on black is supposed to appear darker than the red on white when in reality it's the same shade of red on both the black and the white. There are similar optical illusions which have effects like this, where it seems the featured color is lighter against the white background and darker on the black background.

The above comment was based on the Wikipedia description. The picture above is different from the Wikipedia entry, which bascially has the same type of effect except the one pictured above is much more complex and colorful.