This picture looks like any other colour photo when you first look at it but it's actually an optical illusion, because it's in black and white. 

London-based digital artist Stuart Humphryes has gone viral after sharing the picture with his followers on Twitter, which demonstrates a 'colour assimilation grid illusion'.

A grid of colour lines was overlaid on a black and white picture with Stuart explaining that our brains are trained to 'fill in the gap' of our colour perception  

The brain relays the information it gets from the lines of colours and applies it to the rest of the picture. 

People were mind-blown and many said they wouldn't have noticed it was actually a greyscale picture if they hadn't taken a second look. 

The picture shows a group of teenagers in China posing for a picture. 

It appears that some of them are wearing blue t-shirts, while two are wearing green, one yellow, and another pair, red t-shirts. 

The leaves of the tree in the background also appear to be green. 

But in fact, a closer inspection finds the picture is greyscale, with thin lines of colours overlaid on the picture. 

So what a person sees is not a teenager wearing a green t-shirt. It is a greyscale picture of a teenager, with green lines above their t-shirt.