Visible light is a small part of the electromagnetic spectrum, with wavelengths between about 400 and 700 nanometers. The Wikipedia article on color says that red colored light has wavelengths between 740 and 625 nm; orange, 625 and 590; yellow, 590 and 565; green, 565 and 500; cyan (not blue) 500 and 485; blue, 485 and 440; and violet light has wavelengths between 440 and 380 nanometers. The McGraw-Hill Dictionary of Physics, 3rd edition (New York: McGraw, 2003) says that the parameters are 770 to 622 nanometers for red, 622 to 597 for orange, 597 to 577 for yellow, 577 to 492 for green, 492 to 455 for blue, and 455 to 390 for violet. (There is no indigo in this scheme.) This reference says, correctly, that these are an "approximate range" for each color. The Wikipedia article says:
The color table should not be interpreted as a definite list—the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors is a matter of taste and culture; for example, Newton identified the seven colors red, orange, yellow, green, blue, indigo, and violet . . .
The bottom line is that there is not complete agreement on the definitions of the various colors, or even on their names. Hence, as I remarked in my post on orange, it is no wonder that there is no occurrence of that word in scripture.
Normal humans have three types of cone cells, which are our color receptors, in their retinas. They are most sensitive, respectively, to light with wavelengths of 564, 534 and 420 nanometers. (These light waves can be called yellowish-green, green, and bluish-violet.) Note that the color receptors are not spread evenly over the spectrum. Why? What are the implications? I'm not sure.
What color is perceived is due to more than just the wavelength. Other factors, such as what other colors we are seeing, whether we are seeing reflected or emitted light, and intensity, have an influence.
Do I know that I see green, or even light with a wavelength of 540 nanometers, the same way as you do? No, I don't.
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References added, Feb 18, 2005:
efg's Color Reference library, a page of links on color, categorized.
What is color? from Pantone. Discussion of perception of color, and how color is produced in printing, and on monitors.
"The Importance of Context in Color Perception." Excerpt from Neuroscience, 2nd edition, published by Sinauer.
Page from the American Psychological Association, summarizing research which indicates that ". . . color terms are learned relative to language and culture" (as opposed to being dependent on the properties of human color vision)