Color is perceived by the human eye in a conceptually straightforward way. In our retina there are miniature organs called cones and rods. Cones come in three flavors, one which is "red"-sensitive, one which is "green"-sensitive, and one which is "blue"-sensitive. The reason I have put those words in quotation marks is that every human's actual sensitivity is slightly different, and, even on average, the wavelength difference between the red and green sensitivity is only slight (see this site for detail). However, for most people, red cones are sensitive over a broad range of wavelengths from 0.4 to 0.8 microns and peak at about 0.58 microns; green cones have sensitivity over the same range, but peak at about 0.54 microns and have a relative shift over the range of typically 0.03 microns. The blue cones are sensitive to a much narrower range of wavelengths, peaking at about 0.42 microns (see this graph [reference] -- note that in the graph all cones have been maximized to 1, whereas the blue cone is actually much less sensitive than the other two). The actual wavelength separation between red and green is obviously quite small, and it's easy to see how a minor defect could cause red-green color-blindness.
One curiosity is that the red cones have a "blue leak," which means that they have a small sensitivity to blue light. In fact, the red cone has some sensitivity to wavelengths shorter than even the blue cone can see. This quirk causes the shortest wavelengths visible to humans to appear violet (blue + some red). Remarkably, this also allows for an otherwise impossible "color wheel" in which blue-violet-purple-magenta-red seems a continuous transition, although those colors contain a jump from the shortest to the longest wavelengths we can see.
Color photography and television attempt to stimulate our color perception by using narrow wavelength bands which exploit the regions where a typical human viewer has a large sensitivity difference in their cones. Because of this strategy, photographs often appear too colorful when compared to the real world they're meant to capture. Computer graphics can look cartoonish compared to the real world for the same reason, and typically cyan (green+blue) looks far too bright. Computer monitors generate cyan by directly stimulating your blue and green cones, while in the real world colors between blue and green (turquoise, teal, jade) typically fall into the gap between the green and blue cone sensitivity and look
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