From time to time I come across web pages and groups of people who get irrate about indigo being in the rainbow. There is even a facebook group called “Get Indigo out of the rainbow”. It was Newton who suggested that the rainbow contains seven colours: red, orange, yellow, green, blue, indigo and violet. It has been suggested that, at the time, Newton was trying make some anology with the musical scale and the octave (with its seven intervals) and hence was keen to identify seven colours in the rainbow or visible spectrum. Many modern commentators claim that only six distinct colours can be observed in the rainbow.
Interestingly, the facebook group referred to above would like to eject indigo from the spectrum on the basis that it is not a primary or secondary colour but rather a tertiary colour. The group shows the following colour wheel:
In this so-called painters’ wheel the primary colours are red, yellow and blue and the secondary colours are orange, green and violet. It is argued that since six of the colours in the rainbow are primary or secondary colours in the colour wheel and indigo is not, then indigo has no right to be there. This is wrong on so many levels it is hard to know where to start.
The first thing I would have to say is that this argument seems to ignore the difference between additive and subtractive mixing. Additive mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/ - describes how light is mixed and the additive primaries are red, green and blue. The additive secondaries are cyan, magenta and yellow. Orange is not in sight – and yet surely if we are to make an argument for inclusion in the spectrum based on primaries (and/or secondaries) then it is the additive system that we should be using since the spectrum is emitted light.
The optimal subtractive system primaries are cyan, magenta and yellow (with the secondaries being red, green and blue) though the artists’ colour wheel (which is like the painters’ wheel above) has red, blue and yellow as the primaries.
In my opinion there is nothing special about the colours that we see in the spectrum. Indeed, orange is clearly a mixture of red and yellow and does not seem to me to be a particularly pure colour. I just do not think that arguments to exclude indigo from the spectrum based upon colour wheels or primary colours is valid. That said, I have already mentioned that many people believe that indigo cannot be seen in the spectrum as a separate colour; but this is a phenomenological observation not dogma. I am one of those who believe that indigo and violet cannot be distinguished in the spectrum and therefore I agree with the aims of the facebook group even if I do not agree with their arguments.
The really interesting question is why we see six (or even seven) distinct colour bands in the spectrum when the wavelengths of the spectrum vary smoothly and continuously? I have postulated some possible reasons for this in an earlier post – http://colourware.wordpress.com/2009/07/20/colour-names-affect-consumer-buying/ - but it is far from a complete and convincing explanation. It may explain why we see distinct colours in the rainbow, but why six and why those six in particular. Comments on this would be very very welcome.
Perhaps this, like the conventional (red-yelow-blue) primary colours, reflects the subconscious influence of the opponent hues. Colour is an experience that lies between the poles of redness vs greenness and yellowness vs blueness, so it doesn’t seem at all surprinsing that those four colours should have been singled out by Newton. Theodoric of Freiburg saw these four colours as making up the rainbow, and Aristotle came up with something very similar (red, green, purple, plus yellow by “contrast” of red and green). There’s a complication in that Newton’s “blue” doesn’t seem to be our blue. I know that this question is debated, but Helmholtz for example regarded indigo-blue as having the hue of ultramarine pigment ( which in turn is the hue of our modern RGB “blue”), and introduced the name cyan-blue for the greenish blue part of the spectrum included in Newton’s “blue”. Accepting this correlation, Newton’s three remaining spectral colours would constitute three of the four possible combinations of the unique hues – yellow-red (Newton’s “orange”), green-blue (Newton’s “blue”/cyan) and red-blue (Newton’s “violet”). In opponent terms the seven-hue spectrum is thus R – YR – Y – G – BG – B – RB. I don’t know why yellow-green didn’t get its own name in the spectrum, but there isn’t much in the way of a good name for this hue in general, and perhaps its position at the peak of the photopic sensitivity curve makes us tend to see it as an overlap of yellow and green, instead of a distinct intermediate hue?
Cyan and (“indigo-”) blue look to be reasonably distinct entities in the spectrum to me, despite all that has been wrtitten about Newton making up an extra colour to fit his musical scale. It seems plausible that cyan didn’t get established in the spectrum because there was no room for it in the conventional “artist’s colour wheel”, where there is room for only one “blue” (again, due to the subconscious influence of the opponent hue categories). Now that the name has acquired widespread currency as the blue-green subtractive primary, perhaps we will start seeing it in the spectrum again!
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I red your post today and was surprised. I was born and grew up in Ukraine (former Soviet Union), and was taught in school that colors of rainbow are: Red, Orange, Yellow, Green, Sky Blue (or azure we have a specific name for it, it is similar to #007FFF),Blue, Violet. I know that the same is in Russia.So it’s interesting to me that in some Countries we are taught to see indigo and in others Sky Blue
That is interesting. Many thanks for this comment.
hi everybody
Hi ya
Hi Stephen
I will start shortly a whole series on the rainbow/spectrum on my blog at mycarta.wordpress.com. One of the things I will try to explain and possibly resolve (with more perceptual rainbow colormaps) is the presence of those bands of nearly constant hue. Thanks
Matteo
Well this helped answer my question a little bit. But I am hoping I can find further help on the matter. I teach art to elementary school children and some of them have learned or will learn about the SEVEN colors of the rainbow in science classes. When I was a kid, I have no memory of indigo ever being included in the rainbow. This could just be my bad memory, I’m not sure.
So one, is there a simple way to explain to young children why some people say the rainbow has seven colors and some people say it has six?
Also, why indigo? What made indigo get to be the only intermediate (or tertiary) color in the rainbow? Why not yellow-green?
Thanks
Meg
If I was teaching a group of elementary school children about this I think I would wait until there was a great rainbow outside and then take them outside and look at it and ask them what they see. Do they six colours or do they see seven? Let them decide for themselves rather than impose some sort of dogma about what is meant to be in the rainbow. This also teaches scientific method. Rather than accept something as a ‘fact’ why not observe what is going on and draw your own conclusions.
I have never come across any reasonable explanation yet, as to why we see distinct colours in the rainbow when in fact the wavelength of light varies continuously and smoothly from short wavelengths (bluish purple) to long wavelengths (reddish purple). So why do we see distinct colours and why those particular 6 or 7 colours? I am afraid there is no easy answer to this.
The reason for the colours we see is to do with the response of the eye, which contains colour-sensitive dyes which absorb light of different frequencies. Our colour perception depends upon the amount of light absorbed by each of the dyes.
Indeed, but this does not even begin to explain why we see six or seven distinct colours in the spectrum when the spectrum varies smoothly and continuously in wavelength. The question is why is colour vision categorical in this situation.
I can’t wait for a rainbow, unfortunately. I’ve never seen one anywhere near the school, and But I can show them pictures. Maybe I can create one in the room using a prism. I’ve told them that some people use indigo in the rainbow, but when we color them, we don’t have indigo colored pencils so they can either leave it out or mix the colors themselves. I kind of wanted to use the ROY G BIV song. I guess I’ll just give them a choice.