We see the world in colour (or shades for those who are missing a set of cones). You are reading this from a screen that is projecting coloured light at you, it goes through your eyes and hits the cones and rods in your retina. That triggers a chemical reaction thanks to rhodopsin which uses 11-cis-retinal and light particles to form all-trans-retinal and an electric charge, which sends an ionic signal (that is, not electrons, but charged particles) to the brain.
The brain then looks at these chemical ions and interprets them into what you think you are seeing.
The light particle has a particular frequency which we interpret as light, and an amount of energy we interpret as brightness.
The colour we think we see does not actually exist. You made that up in the hallucination your brain creates that you call vision.
If we examine the light particle (photon) itself, it has no colour, just a certain frequency of vibration in the photonic layer of reality (we think). These frequencies are a certain type of energy amount, but there is no colour. Our eye has several different receptors that react differently depending on what frequency the photon has. The cone we call “red cone” will react to a certain level of strength to different photons around a certain wavelength (the flip side to frequency – they are inversely related). If the photon is dead on the right frequency, it fires at full strength, if it is further away, it fires less vigorously, and if it is too far away, it ignores the photon. So frequencies we call red, orange and yellow will trigger the red cone, but blue won’t. Also infrared won’t. The green cone is also triggered on these frequencies of photon, less for red, lots for green and a bit for blue.
The amount of these signals falls into a certain amount of red, green and blue being sent from that part of the eyeball (the image on your eyeball is upside down) and your brain assigns a value to that combination we call colour. At no point does the red or the orange or the yellow photon have the colour you think you are seeing. That assignment is similar to how computers interpret colour. We assign a level of red, green and blue in number levels, where [0,0,0] is black (no colour) and [full, full, full] is white, [full, 0, 0] is bright red. Do you see it? No? Neither does your eye or your brain.
You navigate the world via a hallucination that you made up based on these values, prompted by some clues out there, that doesn’t look at all like the thing you are fantasising about.
Prompting me to wonder, does the red apple I see look the same to you?
To the best of our ability to know, the red apple triggers a similar pathway and stimulates similar parts of the brain in both of us. That is, a similar part of the brain gets the photon with [full, 0, 0] for the brightest bit of the red apple. What we have no idea is if the conception you have for it is the same as mine.
Consider different languages. I speak English fairly well, so I understand words in English and a word will trigger a meaning in my brain. When I hear a word in a language I don’t speak, it registers as meaningless human speech. The pathway for the English word is fairly well known. The same pathway is travelled for the word in the other language for someone who speaks that language. Yet the meaning they have will likely be different because it means a different thing. We can’t see this bit.
If we had telepathy and I could look in your brain, would you be seeing the equivalent of a different language? Much like speech in different languages also travels a similar pathway and triggers similar parts of the brain for two different people, but is incomprehensible to someone who doesn’t speak that language. We kind of think this is how it works.
So, the colour you see is not real. You made it up.
Yet, it is very useful and that is why we do it.
And that is awesome.