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, which affects almost three million Britons.
Instead, opticians 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 1,000 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).
The quiz of five colours was prompted by another illusion from Optical Express that asked patients to identify whether one square was green or blue.
Two thirds of people quizzed were adamant this shade was green, while the remaining third disagreed – stating it was blue.
However, when asked to name the same colour adjacent to two distinctly blue images, the rate that thought it was green jumped.
Opticians said the colour has the values 0, 122 and 116 on the RGB colour spectrum – meaning the shade is only slightly more green than blue.
SO, WHICH IS BLUE AND WHICH IS GREEN?
COLOUR 1. This shade is blue. R23, G103, B150
COLOUR 2. This shade is green. R0, G122 and B116
COLOUR 3. This shade is blue. R118, G195, B230
COLOUR 4. This shade is green. R113, G208, B197
COLOUR 5. This shade is green. R35, G151, B128