Dotty optical illusion sparks delightfully nerdy debate on colour

We're captivated by optical illusions of all kinds, whether they're accidental or created intentionally to trick our perception. And if they generate a nerdy debate about colour, all the better.

Over on Reddit, people have been left baffled after one user shared the image below and asked what colour the circles are in the image. Are they blue or purple? The question's triggered a big discussion, and it turns out that it all comes back to the fact that the colour purple is an optical illusion in itself.

Are these dots blue or purple? from r/opticalillusions

In the image shared above, the dots are all purple (hex code #6600ff, or electric violet, if we're going to be precise). True blue would be #0000FF, as one person points out. But most viewers report seeing only the dot that they directly focus on as purple, while the others seem to change to a more blue-like hue. Some even say the dots they're not looking at disappear completely.

”Only one is purple to me, but it keeps moving around,” one person writes. “If I look directly at it, it's purple. If I look away at all it's blue,” another person says, equally perplexed “Mine is all light purple with the one I am looking at deep purple. All light purple if I look away,” someone else reports.

Some say the effect changes depending on whether they use the eye comfort shield on their phone. ”With it on there were 9 identical purple dots. When I turned it off, all the dots I wasn't looking at were blue, and only the one I was looking at was a weird/glitchy purple and blue at the same time,” one person writes. It just gets stranger.

It turns out that the baffling optical illusion is one of three that were created for a study on perception of the colour purple. The dots all have a purple colour tone, consisting of a mixture of red and blue with the absence of green, and the background color was chosen to optimise the optical illusion.

The other illusions (b and c above) create similar effects in that the colour purple appears most intensely at the spot the viewer focuses on and fades to a paler hue in peripheral vision.

Optical illusions like this can also be interesting for digital artists and graphics designs because they reveal things about how we perceive different hues (also see our feature on colour theory).

The perception of different colours is a result of the relative activation of three different types of cones in the eye. Blue, green, yellow, orange and red each have a specific spectral wavelength, but purple doesn't exist as a single spectral wavelength in the visible spectrum. Purple is described as a non-spectral colour generated by the brain's visual cortex when L-cones and S-cones are stimulated in a specific combination by the absence of significant signal from the M-cones.

“This constructed character makes purple a fragile and unstable perception that is easily influenced by physiological and contextual factors,” the researcher Hinnerk Schulz-Hildebrandt writes in in When Purple Perceived Only at Fixation: A Fixation and Distance-Dependent Color Illusion, available on Arxiv.

The study concludes that the optical illusion is presumably the result of a combination of three factors: a colour contrast effect, the reception of purple as a non-spectral colour created exclusively through the integration of L and S signals, and the biological distribution of the cones, especially the absence of S-cones in the centre of the fovea. So you now you know. Basically, you can't trust the colour purple.