Try looking very closely at any white area on your computer screen, preferably with a magnifying glass. You will see that each dot on the screen is in fact a cluster of three tiny spots of light : one red, one green, one blue. When the three colours are lit up with the same intensity, the illusion of "whiteness" is created. When only the red and the green spots are lit up, with the same intensity, the screen looks yellow. Try other combinations by gliding your cursor over the above "circuit" of light bulbs ! In fact, all the colours on your computer screen are created by a delicate balance between these three spots of light. How can this be? Because our eyes only contain three types of receptor cells : for red, for green and for blue. But that is another story...