* Scientific study references are indicated in (). These can be found at the bottom of the article, and can be clicked on to access the original reports.
Everywhere we go, our experience of colour is fundamentally made up of mixtures of three primary hues. A printer generates all the colours we see just by blending three coloured inks in varying quantities – these are the colours we see on anything from photographs and shampoo bottle labels, to book covers and juice boxes. Each patch of colour on a TV display or computer screen is actually a tiny cluster of three dots – blue, green, and red – which can produce all the colours visible to the human eye by varying the relative intensities of blue, green, and red light that they emit.
So why is it that specifically three primaries are so central to the way we see and create colour? After all, primaries don’t exist in the natural world, and the number three has no particular meaning. Instead, all the objects and surfaces that surround us reflect lights of millions of different wavelengths, of which our eyes can only see a tiny fraction.
Why is this visible range so limited compared to the true physical diversity of light, and how does this relate to the apparent existence of three primaries?
The wavelengths of light that bounce off objects that we encounter can tell us something useful about these objects, so it’s quite certain that the small spectrum of light that is visible to us is the one that contains wavelengths that give us the most important pieces of information. For example, as an apple ripens, and thus becomes more nutritious, it undergoes chemical changes that make its skin reflect more light of longer wavelengths (around 550-700 nanometres) than before.If your eye has a way of telling apart the wavelengths of light that are reflected by fruit skins as they ripen, then you are able to visually experience the fruit’s transition from green to ripe as a difference in colour. This allows you to decide if a fruit is edible without having to smell or taste it! Animals that benefit from this information (primarily fruit-eaters) have evolved visual systems that are dedicated to discriminating different wavelengths precisely within this useful spectrum of light. Amongst them, the eyes of humans and many primates native to Africa and Asia identify various wavelengths using three types of specialised cone cells (shown below), which allow us to see hues ranging from blue to red.
Knowing this, it might be tempting to assume that the existence of three primaries in our colour vision can be explained by each of our cones seeing a primary colour (blue, green, or red) and somehow blending signals to produce a colour experience. In truth, this isn’t quite accurate, because a cone sees no colour at all. In fact, it’s completely colour-blind. Understanding how cones work, and how multiple cone types work together to overcome each other’s colour-blindness, is key to understanding why all the colours we see can be made with three primary colours.
How cone cells work, and why they’re actually colour-blind
The retina, which is located in the back of the eye, is densely packed with millions of light-sensitive cells – known as rods and cones – which pick up light that enters the eye and begin the process of vision.
These cells are light-sensitive because their membranes are packed with molecules, called opsins, which absorb light as it hits the retina. This light absorption triggers a series of events inside the cell, which regulates how much neurochemical the cell releases to communicate with other cells in the retina that eventually signal to the brain. This enables the brain to deduce how much light is stimulating the eye.
Importantly, the light-absorbing opsin molecules found in cones aren’t equally sensitive to all wavelengths of light in the visible spectrum. Their likelihood of absorbing a packet of light (or photon) and triggering a signal to the brain differs when we look at light of different wavelengths. As an example, let’s take the S-cone, which responds to visible light of the shortest wavelengths that are associated with indigo and violet colour experiences.
The image below is a curve which describes how the S-cone’s sensitivity to light changes depending on the wavelength of light on the retina. Looking at it, we can see that the cone is most sensitive to light of about 455 nanometres wavelength, which means that the opsin molecules in this cone are most likely to absorb light and produce a neurochemical signal to the brain when they’re stimulated by this particular wavelength. The cone is less sensitive to other wavelengths, which means that the further a light’s wavelength is from the cone’s preferred 455 nm, the lower the probability that the cone will absorb and react to a packet of this light. To activate the cone and trigger a signal to the brain, lights of these less preferred wavelengths need to be more intense (so, more packets of light need to be present).
But here, the cone faces a serious dilemma. What if we first stimulate it with some 455 nm light, to which the cone is most sensitive, and then twice as much 470 nm light, to which the cone is half as sensitive? The cone will produce the exact same signal in both cases. When the brain receives these identical signals, it will have no way of finding out what wavelength of light was responsible for activating this cone, and so will see these two lights as identical! This phenomenon is called metamerism.
No matter what cone cell we look at, hundreds of different wavelengths can emulate each other by tricking the cone into producing the same signal, just by changing in intensity. Any time a cone sends the brain a different signal, the brain has no way of seeing if the signal was caused by light being of a different wavelength, or simply becoming more or less intense. A cone’s signal thus tells you nothing about wavelength – it is effectively colour-blind. Even a million cone cells would still be colour-blind, if all of them have the same light sensitivity profile, because all these cones will respond to different wavelengths in the exact same way and thus be equally tricked by different lights. While the colour-blindness of cones might sound somewhat surprising, researchers have known for several decades that extremely rare individuals with only one cone type are usually completely colour-blind (5). An individual whose retina contains only M-cones (previously called green cones) wouldn’t actually see the world in shades of green, but in greyscale. This makes it difficult to say that a single cone in our retina represents a primary colour.
Why colour vision requires at least two cone types to exist
If the retina contains two cone types, each of which is sensitive to a different part of the visible spectrum, then the colour-blindness of each cone type becomes less problematic. This is because light of each wavelength causes the two cone types to produce different signals, as shown below.
Now if the eye encounters lights of the two different wavelengths that trick the first cone into producing an identical signal, the second cone produces different signals for these lights. If we look at how much stronger the signal produced by cone 1 is compared to cone 2, by taking the ratio of the two signals, we can easily tell the difference between when the eye was given 1 packet of 455 nm light and when it was given twice as much 470 nm light.
In fact, this appears to be how the visual system identifies lights of different wavelengths, as the retina harbours specialised cells that calculate the ratios of signals produced by different cone types (1). For anyone interested in understanding this in some detail, check out the image below, which shows how such a ratio might be calculated using two hypothetical cone types (yellow and green).
Since this ratio of cone signals gets communicated to the brain, it is the main source of information that the brain has to help it identify the wavelengths of light that stimulate the eye. This ability to discriminate different wavelengths allows the brain to produce a basic form of colour vision, which is otherwise impossible with just one type of cone.
With two cone types, there is no single wavelength that can be mistaken for another. That’s great, but using two cones is still less than perfect. This is because for any single wavelength, we can find several mixtures of two wavelengths that will produce the same cone signal ratio as that single wavelength. This would prevent the brain from telling the difference between the single wavelength and the mixtures, so that they will all appear identical in colour, despite being physically different! Let’s look at an example below.
Thus, in any visual system with two cone types, the colour experience produced by a light of any wavelength can be precisely matched by several mixtures of two wavelengths. Such a visual system requires two primary colours to produce all the visible hues. Eyes with more cones types are able to distinguish a greater diversity of light mixtures, and are thus capable of experiencing a greater number of colours. Now let’s examine our own visual system, where the retina is a mosaic of millions of densely packed cone cells belonging to three types, each with a slightly different light sensitivity profile.
This three-cone system, known as trichromatic (from Greek tria – three, and khrôma – colour) is at the root of our ability to experience millions of hues. It allows the brain to distinguish lights of single wavelengths and two-wavelength mixtures that would confuse the visual system which possesses only two cones. However, the reason for three primary colours becomes evident when we consider the fundamental limitation of trichromatic vision.
A mixture of just three wavelengths is enough to fool our brains into perceiving it as identical to a single wavelength
Just like a two-cone system perceives plenty of single wavelengths and 2+ wavelength mixtures as identical, the three-cone system is equally confused by mixtures of three wavelengths. Thus, for all wavelengths of visible light, there are multiple three-wavelength mixtures that produce the exact same ratio of signals in our three cone types. As a consequence, the brain, which receives information about these signal ratios, is incapable of telling the difference between the various lights which originally prompted the cones to produce their signals. This means that there is a vast range of physically distinct wavelengths and mixtures of 3+ wavelengths that we perceive as identical in colour.
In light of this, the existence of three primary colours is ultimately rooted in our brain’s inability to discriminate light mixtures that are more complex than three wavelengths – thus, mixing three colours becomes enough to reproduce all visible hues. This is exactly what colour printers and computer screens exploit, as they generate colours by blending three different coloured inks or lights. More primaries are unnecessary, since our retinas couldn’t pick up on the difference anyway.
It is important to keep in mind that the three primaries are only used in the world of colour that we humans create for each other when we paint, print, and display on screens. In the natural world, surfaces reflect an enormous variety of light mixtures composed of numerous wavelengths that we could not even begin to imagine perceiving. While a trichromat might see the bark of the tree below as consisting of subtle variations of beige and pink, rare individuals with four cones in their retinas (such as Concetta Antico, the artist who painted this image) could distinguish the patches of bark that reflect distinct mixtures of multiple wavelengths. To these eyes, tree bark can come alive as a flickering blend of orange, green, brown, and violet patches that we just couldn’t see. The colour world of such humans is instead composed of four primaries, and in my next article, I will examine the fascinating phenomenon of the four-cone observer.
- Diller, L. et al. (2004). L and M cone contributions to the midget and parasol ganglion cell receptive fields of macaque monkey retina. Journal of Neuroscience 24, 1079-1088.
- Dudley, R. (2004). Ethanol, fruit ripening, and the historical origins of human alcoholism in primate frugivory. Integrative and Comparative Biology 44, 315-323.
- Hofer, H. et al. (2005). Organization of the human trichromatic cone mosaic. Journal of Neuroscience 25, 9669-9679.
- Regan, B. C. et al. (2001). Fruits, foliage and the evolution of primate colour vision. Philosophical Transactions of the Royal Society of London 356, 229-283.
- Weale, R. A. (1953). Cone-monochromatism. Journal of Physiology 121, 548-569.