In case it passed you by, last week was the UK's National Eye Health Week. To raise awareness, a laser eye surgery provider called Optical Express ran a test with an apparently easy goal – separate the blue colors from the green.

Warning: it is much, much harder than it first appears. 

The test is an extension of a three-color quiz released in 2016, which involves working out if the central color is blue or green. (You can take it for yourself, here.) This time, the company added two more shades to the mix.

Can you tell which of the five are blue and which are green in the grid below?

First thing's first. Color is not a purely objective thing – we all interpret it differently. (Hence optical illusions like the dress, the chest of drawers, and the Nike gear exist and have the office in fits of disagreement.) So if you see blue but your friend sees green (or vice versa), don't worry. If you can't make out the very slight changes in color, it's not a sign of color vision deficiency or color blindness. It all comes down to brain perception.

As Stephen Hannan, Clinical Services Director at Optical Express, explained, "Light enters the eye and hits the retina, which is the light-sensitive tissue at the back of the eye.

"The light is converted to an electrical signal which travels along the optic nerve to the visual cortex in the brain. The brain makes its own unique interpretation of this electrical signal."

Having said that, there is a technically correct answer. It comes down to the RGB spectrum, which breaks down the red, green, and blue light composition of every color. 

Here are the RGB scores for the five colors above (from left to right):

1. R23, G103, B150 – which makes the shade blue.

2. R0, G122, B116 – which makes the shade green.

3. R118, G195, B230 – which makes the shade blue.

4. R113, G208, B197 – which makes the shade green.

5. R35, G151, B128 – which makes the shade green.

The fourth color on the list is (contradictorily) called Tiffany Blue and it is the very same color trademarked by a certain jewelry retailer. If you got it wrong and said blue, you are not alone – 40 percent of the 1,000 or so people polled by Optical Express also said blue. 

Marrs Green (not on the list) is another color that might have you confused. The deep teal color, reminiscent of aqua environments and named the world's favorite color last year, is (despite its name) equal parts blue and green with an RGB score of R0, G140, B140.