For whatever reason, evolution decided those wavelengths should be overlapping. For example, M cones are most sensitive to 535 nm light, while L cones are most sensitive to 560 nm light. But M cones are still stimulated quite a lot by 560 nm light—around 80% of maximum.
The reason is simple: genes coding the long wave opsins (light-sensitive proteins) in these cones have diverged from copies of the same original gene. The evolution of this is very interesting.
Mammals in general have only two types of cones: presumably they lost full color vision in the age of dinosaurs since they were primarily small nocturnal animals or lived in habitats with very limited light (subterranean, piles of leaves, etc.) Primates are the notable exception, and have evolved the third type of cone, enabling trichromatic color vision, as a result of their fruitarian specialization and co-evolution with the tropical fruit trees (same as birds, actually).
So, what's interesting is that New World and Old World primates evolved this cone independently. In Old World primates the third cone resulted from a gene duplication event on the X chromosome, giving rise to two distinct (but pretty similar) opsin genes, with sensitivity peaks at very close wavelengths. As a note, because these genes sit on the X chromosome, colorblindness (defects in one or both of these genes) is much more likely to happen in males.
New World primates have a single polymorphic opsin gene on the X chromosome, with different alleles coding for different sensitivities. So, only some (heterozygous) females in these species typically have full trichromatic vision, while males and the unlucky homozygous females remain dichromatic.
To me it looked like the circle outline had a shimmering aura, it felt very magical. This was a incredibly delightful experience so I just want to say thanks for posting it.
When the circle was around the halfway point of shrinking the color looked the most vivid for me, so be sure to wait the whole duration.
A bit unrelated but I found this interesting: water is transparent only within a very narrow band of the electromagnetic spectrum, so living organisms evolved sensitivity to that band, and that's what we now call "visible light".
Many years ago [1] I was led to a marvelous site explaining many subtleties of human color perception, including the three cone primaries. The author called this pure-M-cone response "psychedelic aquamarine"; the page is offline but the archive has a capture [2]. I haven't seen this color referred to by that name elsewhere, but I think it's a good name.
I think I can see psychedelic aquamarine and the other cone primaries by closing my eyes and rubbing my eyelids while pressing in gently but firmly.
> If you refused to look at the animation, it’s just a bluish-green background with a red circle on top that slowly shrinks down to nothing. That’s all. But as it shrinks, you should hallucinate a very intense blue-green color around the rim.
I do not believe I have any kind or amount of colorblindness, so imagine my surprise when extremely confused I pulled the image into MS Paint, used the Color Picker tool, and found that indeed, the background has quite a bit of blue in it.
Anyhow, I cannot reproduce the illusion cited. For me the circle just blurs out and I start seeing orange.
I was at a physics conference once and the RGB cable for the presentations was a bit faulty and one of the color channels wouldn’t work, and this presenter’s slides were all tinted cyan. After a bit of jiggling it finally worked and the slides went back to normal, but everyone heard an old professor on the first row go “on no, now they’re purple!”
Painters have been aware of this distinction for years. I encourage interested readers to get a good artist's book on color or just head for your local art store and explore the differences between pthalo and viridian greens (or any of many other surprisingly different tonal clashes).
It is incredible to see a concept going from 'optical table of sensitive equipment fraught with numerous safety concerns' to 'here is a 1 kB svg animation, stare at it for 1 minute' in 3 months.
Incidentally, it’s also a demonstration that you shouldn’t use high contrast in typography. When you start the test you can clearly see the lines of text retained on your retina.
I did a custom combination of yellow outer field, blue inner circle, and got a vibrant purple halo, which is not what I expected. I assumed it would be "yellow++", based on what I know about the human eye's colour sensitivity.
I didn't expect a strong effect, because the overlap between blue and red/green is so much less than the overlap between red and green, but bright purple is close to the opposite of what I expected. I'm genuinely puzzled.
The color (at least to me) doesn't actually look blue-green, but rather cyan-green. This "tertiary" color halfway between cyan and blue (#00FF80) is apparently called "spring green":
I am curious how these work for people with common kinds of colorblindness. The author mentions at the end that they likely don't work for that case, but they don't seem to have spent much time thinking about it.
Would it be possible to generate ones that _would_ work for specific kinds of colorblindness? Or is the entire concept doomed due to the specific way(s) that colorblind eyes are messed up?
"If you’re colorblind, I don’t think these will work, though I’m not sure."
Should work for anomalous trichromats (by far the majority of people with color deficiencies) but probably with less intensity.
"Folks with deuteranomaly have M cones, but they’re shifted to respond more like L cones."
I don't think this is true.
What would the difference between deutan and protan then be?
"Why do you hallucinate that crazy color? I think the red circle saturates the hell out of your red-sensitive L cones. Ordinarily, the green frequencies in the background would stimulate both your green-sensitive M cones and your red-sensitive L cones, due to their overlapping spectra. But the red circle has desensitized your red cones, so you get to experience your M cones firing without your L cones firing as much, and voilà—insane color."
I think only people with missing L cone (Protanopia) or M cone (Deiteranopia) would not experience the phenomenon at all.
Maybe this could be used as a new type of color deficiency test?
This is really cool. Tangentially, it's an example of an important life lesson, "work smarter not harder". To see the impossible color, you could build a super-expensive, super-complicated laser to directly stimulate the exact cells; or you could desensitize the other ones with an optical illusion that works on a personal device (effectively zero cost and minimal complexity since it uses existing technology).
Not to say the laser is a waste, despite the above I'd argue it's very useful. It lets us test how effectively the above actually works, and has other applications.
Woah... did anyone else experience a noticeable desaturated sense of color after coming out of a couple rounds of those things? Like the world was in sepia tone?
The color I saw reminded me a lot of the kind I see when I close my eyes after accidentally looking at something too bright. I wonder if they could be related?
Open the experiment animation and refresh the page multiple times to refresh the countdown while looking at the white pixel (from the same point of view) to get an even more impressive effect.
Is this just my device, or is there no way to use this roll-your-own SVG generator to actually roll your own? I can only pick from a tiny subset of preset colors, most of which seem super random and desaturated and not what I want. There's no FFFF00 yellow, for instance. Is there some way to enter an arbitrary RGB color that I am not seeing? If not, why on Earth write such an interesting article, advertise this custom SVG generator and then build the interface that way? :/
Using psychedelics, specifically 2C-B and LSD, you can also see very saturated colors you don't normally see in daily life. I see very saturated magentas.
Interesting colours coming out of it - a while back I suspected I have https://en.m.wikipedia.org/wiki/Tetrachromacy since I was able to describe colours more vividly that others, and certain plants for me like Verbena have a glow around them.
This reminded me of the works of James Turell. I saw an installation where you observe the sky at dusk or dawn through a circular hole in the ceiling. The ceiling itself was illuminated with saturating colors and there were auras happening around the hole.
What is the animation supposed to be like? I see just a black bar on the left narrowing, but nothing else happens. The red circle and green background and white dot didn’t change. (iOS 26 beta, iPhone 15)
I don’t understand why you need the animation. If you point at the green background after staring the red one for a while, you can see a whole circle of the saturated color.
The green I see around the red circle is exactly the color of a green traffic light bulb when lit, which has a hint of blue to it and is not actually pure green.
It's enough to stare at anything for a few minutes without moving eyes to get similar effects and hallucinations.
We see with good resolution only a small part of our visual field. Perhaps the brain starts to "invent" what's there it we don't give it information by constantly moving eyes.
As a more advanced version, they say that fire kasina practice may produce very interesting visual effects.
New colors without shooting lasers into your eyes
(dynomight.net)567 points by zdw 17 July 2025 | 173 comments
Comments
The reason is simple: genes coding the long wave opsins (light-sensitive proteins) in these cones have diverged from copies of the same original gene. The evolution of this is very interesting.
Mammals in general have only two types of cones: presumably they lost full color vision in the age of dinosaurs since they were primarily small nocturnal animals or lived in habitats with very limited light (subterranean, piles of leaves, etc.) Primates are the notable exception, and have evolved the third type of cone, enabling trichromatic color vision, as a result of their fruitarian specialization and co-evolution with the tropical fruit trees (same as birds, actually).
So, what's interesting is that New World and Old World primates evolved this cone independently. In Old World primates the third cone resulted from a gene duplication event on the X chromosome, giving rise to two distinct (but pretty similar) opsin genes, with sensitivity peaks at very close wavelengths. As a note, because these genes sit on the X chromosome, colorblindness (defects in one or both of these genes) is much more likely to happen in males.
New World primates have a single polymorphic opsin gene on the X chromosome, with different alleles coding for different sensitivities. So, only some (heterozygous) females in these species typically have full trichromatic vision, while males and the unlucky homozygous females remain dichromatic.
Decent wikipedia article on the subject: https://en.wikipedia.org/wiki/Evolution_of_color_vision_in_p...
Types of opsins in vertebrates: https://en.wikipedia.org/wiki/Vertebrate_visual_opsin
When the circle was around the halfway point of shrinking the color looked the most vivid for me, so be sure to wait the whole duration.
http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/imgche/w...
I think I can see psychedelic aquamarine and the other cone primaries by closing my eyes and rubbing my eyelids while pressing in gently but firmly.
[1] https://news.ycombinator.com/item?id=813656
[2] https://web.archive.org/web/20160306132951/https://casa.colo...
I do not believe I have any kind or amount of colorblindness, so imagine my surprise when extremely confused I pulled the image into MS Paint, used the Color Picker tool, and found that indeed, the background has quite a bit of blue in it.
Anyhow, I cannot reproduce the illusion cited. For me the circle just blurs out and I start seeing orange.
Enjoy your forbidden color, you earned it!
I didn't expect a strong effect, because the overlap between blue and red/green is so much less than the overlap between red and green, but bright purple is close to the opposite of what I expected. I'm genuinely puzzled.
https://en.wikipedia.org/wiki/Spring_green
I wonder whether there is a physiological explanation for this.
Would it be possible to generate ones that _would_ work for specific kinds of colorblindness? Or is the entire concept doomed due to the specific way(s) that colorblind eyes are messed up?
Should work for anomalous trichromats (by far the majority of people with color deficiencies) but probably with less intensity.
"Folks with deuteranomaly have M cones, but they’re shifted to respond more like L cones."
I don't think this is true. What would the difference between deutan and protan then be?
"Why do you hallucinate that crazy color? I think the red circle saturates the hell out of your red-sensitive L cones. Ordinarily, the green frequencies in the background would stimulate both your green-sensitive M cones and your red-sensitive L cones, due to their overlapping spectra. But the red circle has desensitized your red cones, so you get to experience your M cones firing without your L cones firing as much, and voilà—insane color."
I think only people with missing L cone (Protanopia) or M cone (Deiteranopia) would not experience the phenomenon at all.
Maybe this could be used as a new type of color deficiency test?
https://www.beloosesky.com/artists/wojciech-fangor
Not to say the laser is a waste, despite the above I'd argue it's very useful. It lets us test how effectively the above actually works, and has other applications.
We see with good resolution only a small part of our visual field. Perhaps the brain starts to "invent" what's there it we don't give it information by constantly moving eyes.
As a more advanced version, they say that fire kasina practice may produce very interesting visual effects.