When the primary colors of light, red, green, and blue are mixed, white light is produced. By looking at the shadows cast when an object blocks one or more of these color components, you can observe both the additive and subtractive processes of color mixing. Instead of subtracting light via an absorbing pigment, the object's shadow "removes" the component color from the "white" light. For example, the yellow shadow is the result of blocking the blue led and only allowing red and green to mix. Similarly, yellow pigment absorbs blue light and reflects red and green. The addition of red and green light makes yellow. The bluish-green shadow - CYAN - is the result of blocking the red light.
Where the cyan and yellow shadow overlaps appears to be green. Why? This small shadow is where both red and blue are blocked. Likewise, when we mix cyan and yellow pigment cyan absorbs red and yellow absorbs blue. The only thing left to be reflected is green. Cyan pigment and yellow pigment both reflect green, so that's what we see. (In white light)
