What is colour? How blue can look white…

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Wat is kleur: kan blauw wit lijken? | Peter Moleman

Translated by Rumia Bose

Most people experience the world as being full of colour. This is the effect of partial reflection of sunlight by objects. Sunlight itself is white: it consists of all the wavelengths of light. Cornflowers reflect light waves with a wavelength of around 450 nanometres (nanometre = one-billionth of a metre), so they have a distinctive colour. Our eyes are so designed that they perceive differences in wavelength. To this purpose we have cones in our retina, as well as rods, which are not colour-sensitive. When the cones are stimulated, then the neurons linked to them transmit electric signals to the visual cortex in the brain.

Wat is kleur: kan blauw wit lijken? Peter Moleman
Figuur 1. Cones (blue) and rods (yellow) in the retina of the eye. (coloured electron-microscope photo)

The human eye has three sorts of cones, sensitive to different wavelengths. A wavelength of 450 nanometres will predominantly stimulate the S cones. A wavelength of 500 nanometres will also stimulate the M cones and change the perceived colour towards green. In this way we can, with the help of just three sorts of cones, distinguish between thousands or even millions of colours of light at wavelengths between approximately 380 and 780 nanometres.

Wat is kleur: kan blauw wit lijken? Peter Moleman
Figure 2. Sensitivity of three sorts of cones to different wavelengths of light. S = short : M = medium and L = long

What’s in a name?

All this sounds rather specific: wavelengths which are perceived in the eyes and transmitted as electric signals to the brain. But the colour that we see and register consciously is not as specific. My wife likes combinations of green and blue in her clothes, furniture and quilts. Some people find that these colours do not go together or even that they clash. Perhaps she sees them differently. That might be so. I might mention a blue coat, and she does not know what I mean. Until she understands that I am talking about that green coat. This naturally is the case when the colour falls between blue and green.
How you name colours is not only important for communicating and preventing misunderstandings. Lera Boroditsky and her co-workers, who study the influence of language on our thought, demonstrate this. In Russian the colour blue does not exist, but two colours, represented by the words “goluboy” for light blue, and “siniy” for darker blue. It now appears that Russians can more easily distinguish between two fairly similar shades of blue if they happen to fall into different categories – one “goluboy” and the other “siniy” – than if they belong to the same category. But if they simultaneously have to repeat words representing numbers then that advantage is lost. In that situation they are not able to use -in their mind- the words for the two shades of blue. Therefore, only if they are allowed to use the different words, then they can more easily distinguish different shades. This means that the names we give colours can influence the subjective experience or interpretation of colours.

Wavelengths are objective, but colours are not

Even the way we see a colour is not objective. That is most clearly illustrated by a hype on internet in early 2015 around “The Dress”.

Wat is kleur: kan blauw wit lijken? Peter Moleman
Source: https://en.wikipedia.org/wiki/The_dress

I don’t know what you see. I see a blue dress with black stripes. But a lot of people see a white dress with gold stripes. How can this be? How we see a colour depends on the lighting. This is not surprising, because the colour of an object is determined by the reflection of the light that falls on it. When you are outdoors, then the light is white, while most artificial light indoors is warmer, with more light of longer wavelengths. Because you can’t tell from the photo where it is taken, your interpretation depends upon your assumption about the source of the light illuminating the dress. People who assume that natural, outdoor light is falling on the dress, see it as white and gold, while people who assume that the dress is illuminated by artificial, indoor light, see it as blue and black. These assumptions are not made consciously. If it were made clear whether the photo was taken indoors or outdoors, then there would be no doubt about the colour of the dress. But how is it possible that, without this information, I see blue and black and another person white and gold? Neither of us knows which light source illuminates the object of the photo. When in doubt, the unconscious brain decides! It has been found that older people and women tend to see white and goldmore often than younger people and men. Possibly this has to do with the predominance of daylight types amongst the elderly and women, and night owls under youth and men. I don’t know if this is true. What is known is that our brain chooses unconsciously on the basis of assumptions. These assumptions vary between individuals. You can also put it this way: your perception of colour rests on a preconceived notion. What is also interesting is that your brain chooses blue and black or white and gold, and nothing in between.
This photo of The Dress became a hype because it is unusual that people vary so much in their perception of the same wavelength. We usually have the same preconceived notions!

Are cornflowers blue?

The colour of cornflowers, with which I started off, will seldom give rise to a discussion, even if they are set in a vase indoors. Well then, are cornflowers blue? You cannot just say this. Cornflowers mainly reflect 450 nanometre wavelengths from white light. We sense these with our eyes, but only link this with the word blue much further on in our brain. The dress reflects light around 450 nanometres, but some people call that white. In figure 2 the sensitivity of the cones and the corresponding wavelengths and colours are named as a given, but this is not really correct.
Some philosophers therefore say that colour does not exist objectively. That is again an exaggeration, because there is a clear distinction between the wavelength of one stripe on the dress with the next. That is objectively measurable, whether you call them white and gold or blue and black. And how you name a colour determines how you treat the colour: Russian or Dutch. Perhaps it is best to say that differences in wavelengths exist, and we name these as colours. Subjective factors plays a role in that process of naming. So, what is colour?


Longden, K. D. (2016). “Central Brain Circuitry for Color-Vision-Modulated Behaviors.” Current biology : CB 26(20): R981-R988.

Lafer-Sousa, R., K. L. Hermann, et al. (2015). “Striking individual differences in color perception uncovered by ‘the dress’ photograph.” Current biology : CB 25(13): R545-546

Daniel C. Dennett (2015) “Why and How Does Consciousness Seem the Way it Seems?” in Open Mind. Thomas Metzinger & Jennifer M. Windt (Eds), Open Mind Group. www.open-mind.net. ISBN: 978-3-95857-102-0. p. 894-923.

Boroditsky, L. (2011). “How language shapes thought.” Scientific American 304(2): 62-65.

Kelber, A. and D. Osorio (2010). “From spectral information to animal colour vision: experiments and concepts.” Proceedings. Biological sciences 277(1688): 1617-1625

Winawer, J., N. Witthoft, et al. (2007). “Russian blues reveal effects of language on color discrimination.” PNAS 104(19): 7780-7785.

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