In 1666, Sir Isaac Newton discovered the foundation for color theory when he observed that white light passing through a prism splits into the seven colors of the visible spectrum: red, orange, yellow, green, blue, indigo and violet. When he passed this rainbow back through a prism, they came together to form white light. Newton concluded that color resides in the mixture of light, not in the object.
Renaissance artist Leonardo da Vinci contributed much to color practice, observing that cool colors cause objects to seem to recede while warm colors cause them to appear to advance. You can use Leonardo's discoveries to create depth in your art. Other more recent color practitioners include Johannes Itten and Albert Munsell.
Additive color theory evolved from Newton's postulation that the human eye mixes visible light reflected by objects to perceive color. The primary colors in additive theory are red orange, middle green and blue violet. In this model, combining all colors results in white.
Subtractive color theory applies to mixing pigments and dyes rather than light. Mixing pigments of different hues reduces the reflected light. In this system, the primaries are red, blue and yellow, and the sum of all colors is black.
Fine artists, interior and fashion designers, and others who work with substantial materials will find subtractive color theory most useful because they deal with the object reflecting the light. Graphic artists and lighting designers benefit most from studying additive theory since their medium is the light itself.
Color is the most obvious element of art. Learning to use it well gives you a bold way to accomplish many design principles--harmony, emphasis, movement and balance--for more compelling designs.