Additive Color refers to the process of adding primary colors (of the given color system) together to produce secondary colors. For example, if the RGB color system is the primary color model, then the primary colors will be red, green and blue. By using an additive color system, a photographer can then add any combination of these colors together to produce any of the secondary colors (magenta, yellow or blue-green, known as cyan).
If all of the primary colors are added together, the resulting light will be white.
Additive color systems work in a directly opposite manner to subtractive color systems that work by absorbing all of the colors except the one that the object appears to be. For instance, a red apple appears red because it absorbs all of the colors of the color spectrum except for red. This negation of the other colors is the mode in which subtractive color systems work.
In the late 1800s, physicist James Clerk Maxwell and photographer Thomas Sutton proved the existence of the additive color system by photographing ribbon under red, green and blue colored filters. The results of each picture rendered an image that had produced an additive effect of all of the colors in the scene.