Light is perceived as white by humans when all three cone cell types of the eye are simultaneously stimulated by equal amounts of red, green, and blue light. Because the addition of these three colors yields white light, the colors red, green, and blue are termed the primary additive colors. This tutorial explores how the three primary additive colors interact with each other, either in pairs or all together.

The tutorial initializes with red, green, and blue filled circles randomly bouncing within the confines of the tutorial window. As the circles cross each other, the corresponding complementary color (cyan, magenta, or yellow) is displayed in the intersected area. When all three circles intersect, the color white is produced. In order to control the movement of the circles, click on the red manual button, which toggles the tutorial between automatic and manual modes. In manual mode, the mouse cursor can be utilized to click and drag the circles anywhere within the confines of the tutorial window. In addition, a blue Auto button appears, which will return the tutorial to automatic mode.

The human eye is sensitive to a narrow band of electromagnetic radiation that lies in the wavelength range between 400 and 700 nanometers, commonly known as the visible light spectrum, which is the only source of color. When combined, all of the 300 or so wavelengths present in visible light, about a third of the total spectral distribution that successfully passes through the Earth's atmosphere, form colorless white light that can be refracted and dispersed into its component colors by means of a prism. The colors red, green, and blue are classically considered the primary colors because they are fundamental to human vision.

When only one or two types of cone cells are stimulated, other (non-white) colors are visualized. For example, if a narrow band of green light (540 to 550 nanometers) is used to stimulate all of the cone cells, only the ones containing green photoreceptors will respond to produce a sensation of seeing the color green. Human visual perception of non-primary additive colors, such as yellow, can arise in one of two ways. If the red and green cone cells are simultaneously stimulated with monochromatic yellow light having a wavelength of 580 nanometers, the cone cell receptors each respond almost equally because their absorption spectral overlap is approximately the same in this region of the visible light spectrum. The same color sensation can be achieved by stimulating the red and green cone cells individually with a mixture of distinct red and green wavelengths selected from regions of the receptor absorption spectra that do not have significant overlap. The result, in both cases, is simultaneous stimulation of red and green cone cells to produce a sensation of yellow color, even though the end is achieved by two different mechanisms. The ability to perceive other colors requires the stimulation of one, two, or all three types of cone cells to a varying degree with the appropriate wavelength palette.

If equal portions of green and blue light are added together, the resulting color is termed cyan. Likewise, equal portions of green and red light produce the color yellow, and equal portions of red and blue light yield the color magenta. The colors cyan, magenta, and yellow are commonly termed the complementary colors because each complements one of the primary colors in a white light mixture. Yellow (red plus green) is the complement of blue because when the two colors are added together white light is produced. Likewise, cyan (green plus blue) is the complement of red, and magenta (red plus blue) is the complement of green light.

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