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Trichromatic Color Theory
Trichromatic color theory is based on the assumption of three primary hues: Red, Green and Blue (RGB). All other colors can be created by a mixture of these.
This theory is based on the system the eye uses of red, green and blue light sensors (cones). Actually, while this is a good approximation, this is not quite the case, as each cone captures a wide distribution of colors (although they capture more of blue, green and green). These are also known as S, M and H, for Short, Medium and High wavelength (blue, green and red, respectively).
The primary RGB colors and secondary CMY colors are shown below:
As the light-emitting RGB system is additive, three spotlights of red, green and blue will show the secondary colors when they overlap:
This can be confusing for people who are used to paints, where the primary colors are red, blue and yellow and they mix together differently. Mixing red, blue and yellow should give black, but the realities of paints often leads to a muddy brown result.
Trichromatic theory was developed first by Thomas Young, who in 1802 suggested that the eye contained three different types of sensors to detect different wavelengths of light. About 50 years later, Hermann von Helmholtz described the eye's cones as each responding to one of short, medium or long wavelengths. The resultant theory is also called the Young-Helmholtz theory of color vision.
The sensitivity of S, M and H (blue, green and red) cones are different, with blue cones being most sensitive (which helps explain why things at night seem blue-tinged). They also cover very different distributions across the light spectrum, with the red and green cones having significant overlap. The red also strays a bit into blue. This can seem rather odd and we may wonder how the colors are differentiated, but the eye and brain somehow manage it (obviously).
Trichromatic theory can be contrasted with the Vision Opponent Process Theory, which is also based on how the eye works but focuses instead on how the color signals are transmitted to the brain.
Televisions, computer monitors, phones and cameras are based on trichromatic principles, in particular that each pixel is represented by three dots (red, green and blue), with the ability to increase the brightness of each dot from off to fully on. When all three are off, we see black (due to the contrast against adjacent dots). When all three are on, we see white (unless we magnify the screen). If all three are set to the same level of partial brightness, we see gray. Many other colors can be shown by varying the brightness of individual dots.
In many digital systems, each dot can have 256 different levels of brightness, due to it being represented in the computer as an 8-bit 'byte' (this is often called '8-bit color'). This means there are 256 x 256 x 256 = 16,777,216 possible colors (this would need a 4096 x 4096 pixel image to show one of each dot). This seems a lot, but the analog eye can see many more. Cameras may capture up to 16-bit color ('high color'), which is about 281,474,980,000,000 colors. This sounds good, but the file size for each picture is much greater than 8-bit. You can even get 24-bit color ('true color') and 48-bit 'deep color'. Given all this, as people can perceive around 2.8 million different hues, there does not seem to be a need for all this variation.
When you are displaying colors, remember how the eye detects these and provide suitable coloration of images.
Young, T. (1802). Bakerian Lecture: On the Theory of Light and Colours. Philosophical Transactions of the Royal Society A. London. 92:12-48.