Basic Rules of Color Theory

Additive Color Mixing

An area that is lit by two flashlights shines more brightly than an area that is lit by only one flashlight. It still shines more brightly if you put a green filter on one flashlight and a red filter on the other one: The light adds up and so the area shines more brightly. This process is called additive color mixing, because the light colors' intensities are added up. The following graphics show three floodlights with red, blue and green light respectively, that are directed towards a bright area in a darkened room:

Additive color mixing

Here you can see that red and green light yield yellow light, that red and blue light yield magenta light and that blue and green light yield cyan light. The colors' brightness also increases. Just as white light can be split up into its colored components (e.g. when passed through a prism), the sum of all those components yields white light again, as shown by the area in the middle.
 
One example for additive color mixing in our everyday life is the computer: A monitor creates colors by illuminating different luminescent materials. Every pixel on the screen consists of three different luminescent materials which are equivalent to the three RGB-colors. The pixels on the PC screen are very tiny and can only be seen with the help of a magnifying glass. On the TV screen, however, they can be seen by the naked eye: When there is a white surface on the TV screen, the three luminescent materials shine equally brightly. The red, green and blue dots can be seen very clearly. When there is a yellow surface on the screen, only the red and green luminescent materials are shining and the blue ones remain dark. A monitor can create an impression of millions of colors with only three primary colors. The additive color mixing procedure is always used when light is to reach the eye directly, i.e. without any reflection by an object.