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The video was shared by Dr Julie Smith, a psychologist who regularly posts advice to her 2.8 million followers.
In the 30 second clip, Dr Smith tells viewers why they see colour the second time they watch the video.
She says: "You're about to experience a negative after-image. I'm going to trick your brain into seeing colour where there is none.
"This video is in black and white, but when you play it back it will be in full colour.
"All you have to do it keep focusing on the cross on my head. By focusing on the cross, your brain is going to do something incredible.
"Over-exposing receptors in your eyes to certain colours causes the brain to see inverted colours when the black and white image appears.
"Now even when it changes keep focusing on the cross."
When the negative colours disappear from the shot and it turns black and white, if you've been focusing on the cross, you should see colours on the image.
But different people seem to notice different colours.
One person asked: "You're wearing a pink shirt, right?"
To which Dr Smith replied: "Yes I am."
Which kind of confuses things when you read the other comments.
One viewer wrote: "I saw purple flowers, your green t-shirt, white trousers, green skirt board, green and blue kid's car."
Hmm, not quite pal.
Another replied: "It worked for me until I looked away from the cross on the play back."
While another is convinced their brain is broken, saying: "It didn't work for me, is something wrong with my brain?"
Someone simply branded the trick in the video 'voodoo magic'.
Another optical illusion claims to show our eyes a colour we have 'never seen before'.
True cyan is a shade of blue that can't be reproduced on mobile phone, TVs and computer monitors.
But an optical illusion, which has been shared on YouTube, works by getting viewers to stare at a white dot in a large red circle for the length of a piece of music.
This, in turn, causes our eyes to see a glowing blue orb, which is true cyan.
But how does it all work?
Well, according to IFL Science, the orb is just an afterimage created by the residual activity of the nerve cells in our eyes.
Most people perceive colour via three different kinds of cone in our retina, each of which responds to a different wavelength - true cyan being 490-520 nm.
After staring at a certain colour for a prolonged period of time, however, the cones picking up that colour will become too overstimulated and therefore desensitized.