We all know the basic colours of the rainbow. The more artistically savvy among us may even be able to divide them up into their three key groups: the primary colours, the secondaries and the tertiaries.

But there are still shades that don’t often occur naturally, and are difficult for things like electronic screens to convey, so they largely manage to evade us.

However, thanks to an optician illusion posted on social media, viewers have had their eyes opened to the dazzling green-blue pigment called “true cyan”.

TikTok user Kate Bacon shared a brief introduction to the colour and instructions on how to trick your brain into seeing it.

“I’m gonna show you a colour that you’ve probably never seen before,” she says in the video.

“It’s called true cyan, and most TVs and monitors aren’t capable of producing this pigment.”

She then displays a red circle with a white dot in the middle set against a blue background.

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Bacon then explains that  you need to stare at the white dot for at least 30 seconds – but the longer the better.

Then, if you shut your eyes tightly, you should see a “glowing orb” in the colour of true cyan.

A number of users have pointed out that it’s impossible to stare at the white dot for 30 seconds because the video only lasts 10.

Fortunately, a longer version of the illusion – posted almost 10 years ago – is available on YouTube. And this one seems to work for most people.

It begins by instructing the viewer to stare at the white dot in the centre of the red circle as the camera slowly pans out.

At the end of the two minutes the viewer should see the colour appear like a halo around the red.

One participant commented: “Now I’m seeing that damn cyan circle everywhere I look.”

Another wrote: “Wow. But now there is a cyan circle that follows my eyes everywhere I go WORTH IT !!!!!”

Meanwhile others said true cyan is now their “absolute favourite” colour.

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