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Additive color theory is generating a mix of with emitted light as opposed to subtractive color which is mixing colors by reflecting light. The customary additive colors are red, green and blue, which are chosen to represent a range of colors that fool the color receptors in the eye into perceiving all possible mixes of colors. An example the customary R G B color set doesn't include any yellow, but the combination of red and green stimulates the eyes sensors in the same way that true yellow light would.
