How many colours are there in a rainbow? Whatever the old children’s rhyme says about purple and pink, the colours usually regarded as visible in a prism, when white light is split into it’s component parts are Red, Orange, Yellow, Green, Blue, Indigo and Violet. Generations have learned this list using mnemonics such as ‘Richard Of York Gave Battle In Vain’ and others, and that does describe the order of the visible light spectrum.
But in reality, the spectrum is just that, a continuum. Light within a certain range of wavelengths, our eyes tend to define as ‘red’, and then slightly shorter than that we perceive as ‘orange’, and so on. Each of us may draw this line slightly differently, and you could argue at length whether a colour is a reddish-orange or an orangey-red, but at some point we call a difference and it’s crossed a line into the next colour along. And the visible part we can see is only a tiny fraction of the electromagnetic spectrum as a whole, from long-wave radio at one end to extremely short wave gamma radiation at the other.
When we look at a rainbow under optimal weather and light conditions, you can sometimes see all the seven traditional colours, although its often hard to discern the difference between blue, indigo and violet – perhaps you can see there is blue and purple, just like in the song (though you won’t usually see pink anywhere, unless you get a beautiful sunset rainbow!). Often the red-orange end of the rainbow seems more vivid to us visually, but every rainbow is unique, and so is every eyeball that perceives it.
The rainbow actually contains light of every shade, every raindrop refracts the light at a slightly different angle to your retina, so actually there are as many colours in the rainbow as there are drops of water in the sky! Now THAT would be something to write a song about