It may sound like a seemingly easy question, being asked to name whether the colour in front of you is blue or green, but a tricky new quiz throws any confidence you had in solving the 50/50 option out of the window.
In the baffling new illusion, opticians have placed five strikingly similar shades of green and blue next to each other.
They say if you can't spot the subtle difference, it does not mean you have a colour vision deficiency, instead they claim the difference is only how the brain perceives the colour.
Optical Express drafted the quiz, which tricked hundreds of people in a survey, as part of National Eye Health Week.
One of the colours the 1000 people quizzed struggled to spot included Tiffany Blue, trademarked by the jewellery retailer. Around two fifths of respondents polled assumed the shade was blue – even those it is closer to green on the RGB spectrum (113, 208, 197).