Red Green Blue

Red.. Green.. Blue... These lights are placed in our theatre in front of a large screen at the back of the stage (cyclorama). By adjusting the amount of light from any of the the three lamps you can create any colour you wish which will provide the background colour.

From Wikipedia:
An RGB color space can be easily understood by thinking of it as "all possible colors" that can be made from three colourants for red, green and blue. Imagine, for example, shining three lights together onto a white wall: one red light, one green light, and one blue light, each with dimmer switches. If only the red light is on, the wall will look red. If only the green light is on, the wall will look green. If the red and green lights are on together, the wall will look yellow. Dim the red light and the wall will become more of a yellow-green. Dim the green light instead, and the wall will become more orange. Bringing up the blue light a bit will cause the orange to become less saturated and more whitish. In all, each setting of the three dimmer switches will produce a different result, either in color or in brightness or both.
The set of all possible results is the gamut defined by those particular color lamps. Swap the red lamp for one of a different brand that is slightly more orange, and there will be slightly different, and more limited gamut, since the set of all colors that can be produced with the three lights will be changed.