Here's another useful illusion using the material technique.

You don't have to just use a color for the shadow. You can use patterns, too. Here I used a Fractal_Sum node to create little spots of color in the shadow. It looks like the glittery ball is throwing pink specular reflections onto the ground.

You'll have to click to see the full size view in order to be able to see the effect clearly.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)