The Hebrew word translated in the KJV as "colors" (or its singular) is ayin (Strong's Concordance #H5869), means "an eye" either figuratively or literally. According to the 1913 Jewish Encyclopedia and several Bible commentaries, ancient Hebrew had no specific term to describe this property of light.
The ancient Israelites certainly knew what colors were as they saw them in Babylonian artwork (see Ezekiel 23:14). They also were aware of the art of their nearby neighbors (Judges 8:26). Scholarship has yet to offer a definitive answer as to the reason why the Hebrew language was deficient in its description of colors.
Although the KJV lists bay, black, blue, brown, crimson, green, grey, hoar, purple, red, scarlet, sorrel, vermilion, white, and yellow, a precise translation of the underlying original language word(s) is difficult. What several of them represent, however, can be gleaned based on where and how they are used in Scripture.
Colors are actually a section of the electromagnetic spectrum (a grouping of charged particles (energy) that move through space at a particular wavelength and frequency) referred to as visible light. The average human eye can perceive wavelengths that are from about 390 nanometers long (violet) to about 700 nanometers (red). A nanometer is one billionth of a meter.
Spectrum of visible light
Isaac Newton gave us the now familiar list of seven wavelengths of light that we can see: Red, Orange, Yellow, Green, Blue, Indigo (a wavelength of light roughly 420 to 450 nanometers long), and Violet. Children in school are sometimes taught to remember this list, from longest wavelength to the shortest, through memorizing the name "Roy G Biv."
The average of them all
Two researchers studying the formation of stars, in 2002, stumbled upon the average color of the known universe. This average was determined after measuring the spectral range of light from at least 200,000 galaxies. Their result, a slightly beige white color (which is the background of the text you are now reading), was dubbed "Cosmic Latte."
Were rainbows always common?
While rainbows and their fascinating display of colors are common occurrences it was not always so. Evidence in the Bible highly suggests that water on the pre-flood earth came not by rain but by ground-based mists and fogs created by the Earth's near total tropical environment (see our article on why there was no rain before the flood).
The absence of sufficiently sized water drops leads to the conclusion that rainbows could not have appeared in the sky until after the rains that brought the great flood. The token of God's promise never to destroy life again by a deluge was not borrowed from an already existing and commonly occurring event. He, instead, infused with meaning the phenomena of colors we call a rainbow.