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
In 2002, two researchers studying the formation of stars stumbled upon the average color of the known universe. It was determined after measuring the spectral range of light from at least 200,000 galaxies. This average, a slightly beige white (which is the background of the text you are now reading), was dubbed "Cosmic Latte."
Meaning of Rainbows
Scripture mentions only three people seeing a rainbow. God used it after the great flood, which Noah saw, as a sign he would never again destroy all mankind by using floodwaters (Genesis 9:13). The Old Testament prophet Ezekiel, in a vision, saw a rainbow above the very throne of God (Ezekiel 1:27 - 28). This bow represented the glory of the Eternal.
The apostle John saw two different rainbows. The first was the same one Ezekiel viewed over the throne of God (Revelation 4:3). The second encompassed the head of a mighty angel who carried a little book containing events to occur in the end time (Revelation 10:1).
Rainbows occur when sunlight refracts and reflects through air containing water droplets, which usually occurs during or right after rain. These bows appear in the part of the sky that is opposite the sun.
While it is true that rainbows contain the seven primary colors delineated by Newton, they also display a whole continuum of light from red to violet and even beyond what our eyes can detect.
One of Isaac Newton's famous experiments, using a prism, proved that white is a composition of all the colors generated by a rainbow. As the sum, white symbolizes (as this series will cover) the complete and perfect nature of God's righteousness and mercy.
What is a moon bow?
Moon bows, also called black rainbows, are a rare phenomenon that occurs only if conditions in a few select areas around the world are just right. They occur during a full moon, when water vapor is in the right place and the nighttime sky is clear. The light from the moon, during these conditions, refracts against the water droplets producing a 'night rainbow.' One of the few places on earth where the right conditions exist for these bows to occur is the Hawaiian Islands.
Were they always common?
While rainbows and their fascinating display of colors are common occurrences today 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 symbolism the phenomena of colors we call a rainbow.