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Colour, it seems, is all around us. From the rich warm hues of a local sunset to the amber traffic light you sped through on the way to class.

We don’t often stop to consider whether the colour we see individually is the same as we believe it to be, so how can we know that we don’t experience a sense of ignorance to the way we see colour?

We already know that this concept can be applied across animals in the world. We see colour through photoreceptor cells found in the retina. There are two types: cones which mediate higher light levels and colour, or rods which mediate low-light and not colour. Most humans, with three photoreceptors—red, green and blue—(trichromats) see more colours than many mammals, for example, dogs or certain monkeys, which only have two photoreceptors sensitive to blue and green light (dichromats). Many birds have a fourth cone which makes them tetrachromats, giving them the ability to see in ultraviolet. However, the mantis shrimp really puts us all to shame. It has 12 to 16 types of photoreceptors in comparison to our three. Along with being able to see in ultraviolet, which humans can’t, it has the ability to perceive an enormous wealth of colours unimaginable to us.

But a question we have to ask ourselves is: what even is colour?

The Oxford dictionary defines colour as, “the property possessed by an object of producing different sensations on the eye as a result of the way it reflects or emits light.”

Nowhere does this definition say that colour is an objective, measurable truth like gravity or distance. In fact, while colour seems entirely real, it’s a ruse. It’s a matter of photoreceptors in our eyes seeing light rays from the electromagnetic spectrum and perceiving them as a certain value—like a verdant or vermillion. In other words, it exists in our heads, but not in the real world—psychophysical rather than physical. As artist and educator Josef Albers put it: “Every perception of colour is an illusion.”

We can go about our merry lives ignoring this idea, but when we can’t decide whether a dress is blue and black, or white and gold, it does make us wonder.

In a philosophical sense, we see the world through elements of subjectivity, and an “explanatory gap”, a disparity between the physicality of the world and what we perceive may be the biggest setback in being able to understand whether your red is my blue.

While the general consensus of scientists in the past would have been that we all see colour the same, University of Washington colour vision scientist Jay Neitz, whose work also includes the potential use of gene therapy experiments to cure colour blindness, suggests otherwise.

“I would say recent experiments lead us down a road that we don’t all see the same colours,” he said.

We don’t have a definitive answer on whether we see colour the same across the board, but that hasn’t stopped scientists from hammering away at the idea. Brooklyn College psychology professor Israel Abramov found that people with XX and those with XY chromosomes with normal colour vision actually see colour slightly differently.

“There were relatively small but significant, differences between males and females in the hue sensations elicited by almost the entire spectrum,” the paper stated.

The team’s work suggests that people with XX chromosomes tend to see subtle differences in hues where people with XY chromosomes do not; for example, being able to tell the difference between two shades of lipstick or paint which may look the same to someone with XY chromosomes. Additionally, people with XY chromosomes could perceive colours as warmer than people with XX, as people with XY chromosomes “require a slightly longer [visually warmer] wavelength than females in order to experience the same hue.”

Another inquiry into colour perception has found that a rare percentage of people with XX chromosomes have an extra photoreceptor and can see colours beyond the normal scope of vision. These tetrachromats, with their extra fourth cone, are estimated to be able to perceive a hundred extra variants to normal colours available to most humans. Gabriele Jordan from the University of Newcastle outlines a potential reason behind this which is that red and green photoreceptors are carried by the X chromosome. For people with two X chromosomes, there is the possibility of carrying two different versions of the gene, both perceptive to slightly different areas on the colour spectrum. However, this is not to say that people with XY chromosomes have no chance of being tetrachromats—it’s just less likely. Perhaps like the X-Men, people with tetrachromacy are superhumans of colour.

While colour perceptions may vary slightly from person to person. A day-to-day aspect of life which has been around for billions of years is a sunset, something that often leads us to question why the sky changes colour.

The sky appears a certain hue due to the way sunlight travelling to earth scatters in the atmosphere. This is when molecules in the air change the direction of light rays. When the sun is high above the horizon during the day, it appears blue because air particles scatter more short-wavelength blue light from the sun than the other, longer wavelengths such as red or orange. Thinking back to our Oxford definition of colour, the cooler blue wavelengths are reflected, and the warmer hues are emitted.

During sunset, however, the sky is lower on the horizon and needs to take a longer path to pass through more air particles in order to get to our eyes. The longer path that light takes during sunset results in shorter wavelengths like blue and violet to disperse earlier in the light’s journey and for the longer red, orange, yellow wavelengths to reach your eye.

Like many things in life, colour is a beautiful illusion. So, next time you’re looking at a sunset, thinking about taking your phone out and posting another extremely original picture of the sky to your saturated social media, consider the absurdity of how the exact scene you see may be completely different for someone else, and a picture won’t change that.