Additive mixing of color takes place when two or more light beams with different colors are superimposed on a screen or directly on the retina of the observing eye. One example is theater stage illumination where several colored light projectors illuminate the same region. Another example is the well-known rotating top with a disc of colors. If all colors of the spectrum are in the disc, a neutral gray color is observed when the top is rotating. A third example is in color television screens, or in computer screens. Extremely small red, green, and blue dots are produced on the screen. The relative intensities of these colors produce a wide range colors for the eye.
The CIE chromaticity diagram is a useful tool for predicting the color produced by different color additive mixtures. Consider two light fields with spectral powers P1(λ) and P2(λ).