Purple is not on the rainbow. So how can we see it?

Every time you see the colour purple, your brain is executing a gorgeous hack of your visual system.

Is purple really colour? Yes … but

In the last #NerdNews I told you that purple tomatoes will be hitting Aussie supermarket shelves later this year.

The day before, I’d seen a headline in Popular Mechanics: “Purple is not really a colour.”

What’s going on here?

Let’s take a few minutes to marvel at how our incredible visual system allows us to see this “non‑colour”.

I can sing a rainbow

Ask a kid to list the colours of the rainbow and you’ll usually get some version of: red, orange, yellow, green, blue, indigo, violet.

In my day we called it Roy G Biv. Roy’s name spelt out the stripes that come from white sunlight being split by raindrops (or a glass prism) into different wavelengths.

This spread of wavelengths goes from long (red) to short (violet).Each band is a chunk of the visible spectrum where light of similar wavelengths is being bent to the same place on your retina.If you scan along that spectrum, you pass smoothly through reds, oranges, yellows, greens, blues and into violets.

But you never hit “purple” as its own band.

So if purple isn’t in this natural rainbow of pure light, how on earth do we see it splashed all over logos, clothing and, soon, supermarket tomatoes?

How cool are cones

On your retina you have three kinds of colour‑sensing cells called cones, each tuned to a different slice of the spectrum: one responds best to short wavelengths (blue‑ish), one to medium (green‑ish) and one to long (red‑ish).

Any light that hits your eye will usually tickle more than one cone type at once, and your brain reads the pattern of activity across all three to decide what colour you’re seeing.

Colour isn’t stored in the eye like paint in a tube; it’s a comparison problem the brain solves on the fly, millions of times a second.

Some people have one cone type that doesn’t work properly or is missing entirely, which scrambles those comparisons and might show up as red‑green colour blindness.

A small number of women have a fourth kind of cone, potentially giving them access to fine colour differences the rest of us literally cannot imagine.

The colour purple

Say you’re looking at a piece of purple cardboard under ordinary white light.

The dye molecules in that cardboard are absorbing a lot of the green‑ish middle wavelengths and bouncing back more light from the red end and the blue‑violet end.

That mixed light hits your eye and strongly excites your “red” cones and your “blue” cones, while leaving your “green” cones relatively quieter.

Your brain sees this odd “red + blue, not much green” pattern and, instead of labelling it red or blue, it creates a new, stable experience: purple.

Voila; you see a purple piece of cardboard.

Seeing is believing

Once you see the trick for purple, it’s hard not to be impressed by the whole system.

Brown, pink, magenta and many other familiar colours are also brain‑made constructions, built from mixtures of wavelengths plus context, not single “pure” colours of light.

Your visual system takes a messy spray of photons bouncing off objects and turns it into a rich, continuous world of colour that feels immediate and effortless.

The fact that there’s no purple in a simple rainbow, yet you can spot a purple tomato at a glance in a busy supermarket, is a quiet reminder of the power and creativity contained in your brain’s colour machinery.