Thought Leadership

Every color of the rainbow

By Colin Walls

What do you do when you see a rainbow? I will tell you what I do: I just stand and stare. I have no idea how many I have seen in my life – it must be many hundreds – but I still stop. If I am inside the house and observe that it is raining and the sun is shining [and the angle of the sun is low enough], I rush outside to see whether there is a rainbow. Of course, on a good day, I will also see a fainter secondary rainbow. Once or twice, I have seen the faintest hint of a tertiary too. Even though I know that all I am seeing is the physics of light displayed on a grand scale, I still find a rainbow awe inspiring.

Pondering rainbows has got me thinking about colours in general …

Most humans see in color. This is not true for all animals. Amongst mammals, it is essentially true to say that carnivores do see in color with high resolution, but require plenty of light. Herbivores have no color, and less precision, but can detect movement very well and work in low light. We have hybrid mammalian vision: Ahead we have a carnivore’s capabilities, whereas our peripheral vision is more like a herbivore. To most humans, color is important. We have favorite colors and like particular combinations. As a photographer, I think a lot about color [or lack thereof] in a picture.

How many colors are there? This is a question with no specific answer. As color is just frequency of light, there is effectively an infinite number of colors. The human eye can distinguish around 100 different colours. A JPEG image from a digital camera can contain about 16 million colours [24 bits]. A RAW image from a camera can represent 70 billion colors [36 bits] or maybe more. We think of a rainbow as having 7 colors, although many people argue that 6 would be enough to broadly describe it.


It is curious, but colors contribute a lot to our language and thoughts, sometimes in surprising ways:

Red: an indication of danger; the setting/rising sun; blood; embarrassment; heat; communism.

Orange: a fruit; a political group in Ireland; historically well known cellphone company in Europe.

Yellow: quarantine; cowardice; jaundice; snow [Frank Zappa]; bananas.

Green: nature; ecology; permission/safety [traffic lights]; naïvity; mould; apples; gardening [“green fingers”].

Blue: sky/sea [“blue sky thinking”, “out of the blue”]; music [The Blues]; sadness; coldness; pornography.

Indigo: a dye.

Violet: a flower or woman’s name.

There are handy mnemonics to remember the sequence of colors. I was always taught “Robert of York gained battles in vain”. I have a new one that is too rude to publish here [email me if you are curious]. Do you have a good mnemonic?

Incidentally, when we see an arch of light across the sky, we call it a “rainbow”. Seen from above [which i have a couple of times], a rainbow can form a complete circle and it is called a “halo”. If the rainbow is formed as a result of something other than rain [like a fountain or a waterfall], it is called a “glory”.

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This article first appeared on the Siemens Digital Industries Software blog at