Drag a piece across the board. Wherever it blocks a light, it leaves a shadow behind it.
Slide a light along the border to swing its shadows around. Tap it to change its color or switch it off.
A shadow is still lit by the other lights, so it takes their color. Where no light reaches, it turns black.
In Play, match the picture above the board. Tap it to see it bigger; when everything is close, it clicks into place.