A classic optical illusion called 'true cyan colour' has become popular on TikTok It involves staring at a white dot inside a larger red circle for up to three minutes Doing so, then closing your eyes tightly should reveal a glowing 'true cyan' orb This is said to be a version of the colour cyan that computer screens can't show It is actually the result of nerve cells that 'see red' becoming overwhelmed and when you open your eyes other cells take over showing a reverse 'afterimage'