We usually say there are seven colours in a rainbow but this is a bit of an understatement. A rainbow is an arched spectrum of light that we see in the sky when the Sun shines onto raindrops. It has thousands of different colours.
Without getting too heavily into science, let's just look at some basic things about colour:
We might remember being taught at school that there are three primary colours (red, yellow and blue), and mixing these, we can get the three secondary colours (red + yellow = orange, yellow + blue = green, and blue + red = violet). These six colours are shown below.
Each colour has a wavelength, as we see in the chart on the right. (Beyond the red 780 nanometres (nm) mark, we have infrared, which we cannot normally see with the unaided eye. And beyond the violet 390 nm, we have ultraviolet, which again, is outside our normal vision1.) Mixing the primary and secondary colours gives us more colours (for example red + orange = salmon) but the wavelengths of these tertiary colours are not nearly as wide, and therefore not as visible as the six cardinal colours.
So there we have it. Only six main colours in the rainbow.
So what is the seventh colour? The answer is of course indigo, a tertiary mixture of blue and its neighbouring colour violet. Indigo has a very narrow band (around 450-440 nm), and as such, has very little reason for being included in our description of the rainbow.
So why is indigo there? Well, when astronomer, natural philosopher and Unitarianist Isaac Newton divided up the visible spectrum, he decided there should be seven colours to link them with the seven 'planets' known at that time. This was one of his missions in life: to show an integrated and harmonious cosmos.
Indigo is a rather drab mixture of blue and violet. Maybe a nice salmon colour, mixing red and orange, could have been chosen. But Newton was keen to show the relationship between colour and music. He felt that it was no coincidence that the musical diatonic scale had seven notes (do-re-mi-fa-so-la-ti), although he seems to have ignored the important difference that the musical scale is cyclic (i.e. it repeats at each octave) whereas the colour spectrum is not.
Nevertheless, he decided that red corresponded to D on the musical scale and therefore indigo became B (going down the colours and up the scale). Did the frequency of that note give Newton a 1931 Duke Ellington style Mood Indigo?
There have been many theories that pitch and colour are mysteriously related. To name a few, the romantic composers Alexander Scriabin (1872-1915), Hector Berlioz (1803-1869), Claude Debussy (1862-1918) and Richard Wagner (1813-1883), all had interesting ideas. (See the excellent, easy-to-read, discussion on the relationship between pitch and colour at www.mathpages.com/home/kmath578/kmath578.htm.)
After World War II, there was a medal given to soldiers involved in the D-Day operation. This was the Operation Overlord Medal and the rainbow colours were used for its ribbon. The rainbow ribbon was also used for the First World War Victory Medal. Now, of course, not many people can remember what the victory was, who was fighting, and why. That makes the 21,241,000 wounded and 8,281,250 killed2 seem more futile than victorious.
(See also Rainbow Ribbon.)
The rainbow is also symbolic of a halo3 and since the time of Adam and Eve, the rainbow has been considered both beautiful and mysterious. Small wonder that rainbows appear as good omens in mythology. To the ancient Greeks, the rainbow was a path leading to heaven. We find similar beliefs in China, Africa, India and Europe. And in Ireland, the little leprechauns are said to have hidden their pot of gold coins at the end of the rainbow (but nobody knows why).
The arc of the rainbow looks like a bow, facing the heavens. Turning the bow on oneself is believed to have been an ancient symbol to declare a cessation of hostilities.
In the Old Testament, the rainbow is a symbol of the covenant between God and man after the great flush4. The Rainbow Cross indicates that the same God Who saved Noah and his family, now extends that salvation to us all. (The Ethiopians actually honour seven covenants, the second of which is the rainbow of Noah5)
The meaning of the Christian cross is well-known. But the Rainbow Cross has recently acquired a more specific meaning.
The rainbow shows us the full spectrum of colours and is universally accepted as being good and natural. It is used in hippy peace designs. It is used by Jesse Jackson's Rainbow Coalition to show that all colours are needed to make the universe whole. It shows that the world is composed of diverse people, in terms of race, colour and ability. It is used by people who have different sexual orientations, particularly gays.
On 27th November 1978, Harvey Milk, the city's first openly gay supervisor, was assassinated along with Mayor George Moscone.6 The turnout for the 1979 San Francisco Gay and Lesbian Pride Parade7 was massive, due to the publicity and anger over Harvey Milk's killing. The Parade Committee decided to use a rainbow flag, designed by Gilbert Baker, as a symbol of solidarity for gay people. For the grand parade, they removed the indigo stripe and then divided the flag into two: red, orange and yellow stripes on one side of the street; green, blue and violet on the other.
Since then, the six-colour flag has been the symbol for gay rights and equality around the world. Gay Christians have used this symbol to make the Rainbow Cross. (See also the St. Sebastian Cross.)
Whatever your view might be about Homosexuals and the Church, the rainbow is generally considered, like the halo, a 'good thing' and it often appears on, or with, a cross as a peace symbol. It reminds us of God's love and purity.8