There are two modes of color that generally define the visual world: Additive and Subtractive. In Additive, all colors combine to create white. Additive is the color system of nature. A flower garden, a landscape, a rainbow are everyday examples. So is projected light such as from televisions and computer screens. Additive colors are made up of three primaries: red, green and blue.
Subtractive color on the other hand is basically man-made. Think about ink and paint. Nature is somewhat involved but is not the main driver. When subtractive colors are mixed, they darken – even to the point of appearing black. Subtractive colors are made up of three primaries: yellow, red and blue. When subtractive colors are mixed they tend to darken because light is being reduced. When all colors are combined in additive color, white is the result because light is being added.