Why magenta doesn't appear in the rainbow

Yes, red and blue cones both fire when you look at violet rainbow light. Even though the wavelength is shorter than blue, the red cones do actually fire. They are not limited to only firing on the longer red wavelength.


Not really. You’re arguing semantics. There is a wavelength of light that produces the sensation of yellow. There is no wavelength of light that produces the sensation of magenta - you can only do it by mixing two different wavelengths. (You can also produce a sensation of yellow by mixing two different wavelengths, but this will only work if the viewer is a trichromat, which most of us are)

When he says “color” he means something like “a pure wavelength of visible light”, when you say color, you’re talking about something like the specific sensations we get. You’re just talking past each other.


There is indeed a spectral color that stmulates blue and red cones. We use the word “violet” for it.

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Violet is not magenta. The mechanism is similar, but the values are not.

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Sokath, his eyes uncovered!


Correct. Also “brown” and “gray” are not spectral colors, but are combinations of 2 or more wavelengths. Magenta is the word we use to mean a combination of red and blue light. Magenta is as much a “color” as yellow and cyan.

However the guy in the video implies that there is no spectral “rainbow” color that stimulates both blue and red cones. That is incorrect. Violet light does just that.

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@SheiffFatman & @Archvillain
(disclaimer: apologies in advance since i am reply to two people and making some additional points of my own, so please don’t misunderstand that i’m thinking you think or don’t think something personally…cheers.)

you are speaking strictly in additive color theory terms, which is a partial view not a complete picture. Please look up subtractive color theory. It isn’t just semantics.

-you have the frequencies of the light hitting a surface.
-the surface absorbs and reflects various frequencies.
-the reflected light then enters the eye where it stimulates a combination of various cones and rods which produces a signal which is set to the brain for interpretation.
-even how the brain interprets the colors is subjective based on ambient light and recent previous color inputs. (try staring at a green dot for a few minutes then look around the room.)

saying magenta isn’t a color because it requires a combination of light frequencies, is like saying yellow isn’t a color because it requires the stimulation of several different cone types. It really truly isn’t that simple as one or the other. are you defining color as a property of the object (absorption/reflection), as a property of the light being reflected, as how the eye processes that light, as how the brain interprets that signal from the eyes? They are all part of the complete color picture, which is why physicists speak in wavelength not subjective color labels. The light only model does not a color make.

even black isn’t the absence of all color. true black is, but you can’t see true black. really when you look at a “black” object you are seeing a lot of different really dark colors.

Not to mention how the brain interprets metallic, opalescent, or highly reflective surfaces. If you ask people what color a mirror is they will say silver, which is technically true but also false. look in a mirror, all you see is a reflection of every color in the image in the mirror.

etc. etc.

not so simple as “light frequencies”

color theory is a really fun subject to study because it is so multi faceted and goes so deep.

if you shine red blue and green lights at something white or your eye you can make all colors.

if you look through cyan magenta and yellow transparent films at something white or reflect white light off of combinations of cyan magenta and yellow pigments you can make all colors.

both are equally valid and inseparable aspects of what we define as color.

this is why your screen is RGB and when you go to print something it is CMYK.

Say what you like about the imprecision of some of the headlines here, when they yield responses as excellent as this one… well it’s why I come.


I’m well aware of color theory, and what you’re doing is semantics - you’re quite clearly talking about different things. You’re talking past each other. Your points aren’t wrong and they don’t negate his.

i’d like to think that i was talking about BOTH aspects of the same thing. not mistakenly getting hung up on one or the other. BOTH are required for a complete understanding, which is all i’ve been pointing out.

A shout out to: Um Shits Complex Yo :slight_smile:

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When did we start defining a colour by “only if it has one wavelength”? Did I miss a memo?

/Aside from: what if the quale of magenta is what a bee sees when it sees red?

//typos! typos…

It may well be the case that I know as much or more about color than you do, but unlike you I really wasn’t bothered by the way the guy explains light - it seemed quite clear to me what he meant (though I skipped past a few bits). I didn’t perceive it as an attack of ignorance, or as meaning he doesn’t understand subtractive color, or that the explanation demands a countering, or that your response corrects or invalidates his way of explaining things. You expound on the topic along the way, which is a great contribution for people curious about the topics, but I think your frustration with his explanation stems from difference in perspective or weighting rather than there being a Serious Problem That Needs Correction. (We geeks tend to do that with things in our bailiwick)

Anyway, good work. Carry on! :smile:

We didn’t define color. The memo was that as long as someone explains their terms, it can be disingenuous or ungenerous to ignore that information when interpreting their words. Personally I think the context and explanations he provided were sufficiently clear (even if they annoyed some :smile: )

Magenta you say…


Define color.

“Absence of green” presumes an RGB color space. That’s because humans have RGB color receptors. Except for the odd mutant (mostly women) who also have a Y. And except for mantis shrimp, who have 12 distinct color receptors.

Color is probably very idiosyncratic. Even what we recognize as red, green and blue is not physics red, green and blue. Try it yourself. Find the best red, green and blue paints you can find, mix them, and you’ll get brown.Blue, especially, is generally adulterated with a lot of other colors. I learned this once in the middle of a class on color.

Dammit, I had to scroll all of the way down to the bottom of the page to see that someone else had already done it. :smile:

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Strictly speaking, colours don’t exist in physics at all. Physics has wavelengths.

Our retinas convert certain wavelengths into signals, throws those over to the brain, which then interprets them, and the result of that interpretation is the mental construct we perceive as colour. It’s not a passive interpretation either… look at the “what colour is this dress” phenominon to see one example of how we’re subconsciously processing the data.

Colours are the end result of that interpretive process, not the original input.


The amount of metamerism in this thread is too damn high!

I finally watched the video. So many straw dogs!! Arrrghhh!

The rainbow is supposed to be the full spectrum of colours. Uh, no, it’s the full spectrum of visible wavelengths.

The former name for purple is magenta!? Wha??? Another memo I missed? The former name for mercury is quicksilver, and yet there’s no silver in there! What Is going on?

He was so close: the colour-perception system wraps around red and blue and ‘creates’ the colour in between the two, magenta. (Why didn’t he talk about the colour wheel?

If I use the information-depleting sophism Mr. Mould uses, I can claim that there’s no such thing as white because there’s no white wavelength, no?

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