The 'weird' optical illusion baffling the internet: What colour do you see?

British psychologist Dr Julie Smith posted a black and white video of herself sitting outdoors with a white cross on her head. Photo / @drjuliesmith

We all remember the blue vs gold dress debate but there's a new optical illusion that is making people feel like their "brain is broken".

British psychologist Dr Julie Smith posted a black and white video of herself sitting outdoors with a white cross on her head.

"You're about to experience a negative after image," Smith begins the video. "I'm going to trick your brain into seeing colour where there is none."

Smith switched the black and white video to a contrasted version and told viewers to "keep focusing on the cross on my head".

"By focusing on the cross your brain is going to do something incredible. Overexposing the 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."

If you've been following Smith's instructions properly, when the video switches back to black and white the image appears in warm or muted colours.

The video, which has been viewed more than 413,000 times, stunned Smith's followers who dubbed it both "weird and cool".

"I think my brain is broken," one person commented.

"I've rewatched this so many times, my mind is blown," one viewer wrote.

"That was crazy! It was only for a split second but wow," another said.

But others complained they hadn't been able to see the optical illusion at all and the video made their "head hurt".

"What does it mean if you can't see colour? Broken brain?" one person asked.

Last week a photo of a girl seemingly stuck in a footpath went viral after it was shared to Reddit.

"My daughter, where's the rest of her?" the caption from Reddit user MK24ever reads. "Oh I see, do you?"

"This is making my brain hurt … I just can't see it," one person tweeted.

"Oh wow that took a minute," another commented.

After staring at the image for long enough most people were able to work out what was really going on: the girl was actually standing behind a low wall made of the same material as the footpath.