The Six Colours of a Rainbow

We’ve seen a lot of beautifully drawn rainbows recently but what colours should you use. It’s a simple matter of physics, but not everyone has seen rainbows the same way, and when Sir Isaac Newton got it wrong we all followed. This is a potted history of how rainbows have been described over millennia.

homer's rainbow

About 3000 years ago, Homer, an Ancient Greek epic poet and philosopher, described rainbows as being a single colour – purple. Maybe he was severely colour blind or more interested in poetry than painting.

a rainbow of three colours

About 2500 years ago, another Greek philosopher called Xenophanes of Colofon identified two more colours. He added red and a yellowy-green to the purple. Still not much like a real rainbow.

The European Renaissance
Some 500 years ago, during the European Renaissance pretty much everyone ‘in the know’ agreed there were four colours, red, yellow, green and blue. But they got a lot of things wrong in the Renaissance period.

It’s worth remembering that orange had only just been named as a colour – before then it just meant an orange – and the colour was referred to as a yellow-red.

The Enlightenment
It began to make a little more sense around 400 years ago at the start of the Enlightenment, known as ‘The Age of Reason’. Science began to challenge some long held beliefs. Europeans expanded the rainbow to five colours – red, yellow, green, blue and purple.

Science takes over
A major change came in 1665 when Sir Isaac Newton split light using a prism (he bought it at a county fair) to make a rainbow on the wall of his room. He got his assistant, under instruction, to mark the divisions of colour with a pencil.

A incorrect correlation
Newton believed there must be a connection between the number of colours in a rainbow and the number of notes in a musical scale (he was wrong). We still use the names he gave those seven colours – Red, Orange, Yellow, Green, Blue, Indigo and Violet.

But colours sometimes change name
Newton probably intended for the seven colours of a rainbow to look more like this. That one he called indigo was almost certainly dark blue – he would have known Indigo as a dye that comes from a plant called Indigofera – and his blue might well have been a paler greenish blue.

The primary colours of light
The three primary colours of light are Red, Green and Blue. Mixing them is called additive colour mixing because the more of the primary colours you shine in one place, the closer you get to white. (its not the same as mixing paint)

Red and Green combine to make Yellow.
Green and Blue combine to make Cyan (a pale blue).
Red and Blue combine to make Magenta.

You may recognise the colours, or even just the letters RGB, as they are the three colours used in televisions, computers, smartphones and stage lighting. The three secondary colours may also be familiar as the colours of the inks in a inkjet printer.

A more accurate rainbow
So it might make sense to use these colours for our rainbow drawings. Of course we’d have to think of a new mnemonic to remember them. Roy G BIV (USA) or Richard Of York Gave Battle In Vain (UK) would no longer work.

How about a new mnemonic for Red Yellow Green Cyan Blue Magenta?

Rainbows You Gladly Colour By Magic?

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