Green is an interesting colour that sits between blue and yellow on the visible spectrum. Unlike red, blue, and yellow – which are considered primary colours in many colour models – green is seen as a secondary or tertiary colour as it can be created by mixing two primary colours together. However, some theories and models consider green as a fourth primary colour alongside red, blue and yellow. So why is this? Let’s take a closer look at the science and theory behind green’s status as a colour.
The origins of primary colours
The concept of primary colours originates from the creation of pigments for painting. When artists mix together the primary pigment colours of red, yellow and blue, they are able to create all other colours on the visible spectrum. However, when we look at light and illumination, the primary colours are different. Red, green and blue (RGB) are the primary colours of light, meaning all other colours can be created by mixing those three. This is because the primary pigment colours actually absorb certain wavelengths of light rather than emit them.
So when we talk about primary colours, we have to specify whether we are referring to pigment primaries or light primaries. The colour green sits somewhere in the middle, as it can be created by mixing blue and yellow pigments, but is also a primary colour of light alongside red and blue.
The primary colour theory of Hermann von Helmholtz
In the 1850s, scientist Hermann von Helmholtz advanced a new theory of primary colours based on the physiology of the eye. He identified the three types of colour receptor cells in the retina – which roughly correspond to red, green and blue – and proposed that all colours are perceived by some combination of these three receptors.
Based on this theory, Helmholtz designated red, green and blue/violet as the primary colours. So unlike traditional RYB pigment theory, he classified green as a primary colour due to its role in colour vision. This also matches up with the primary colours of additive light mixing.
Helmholtz’s theory was revolutionary at the time and laid important groundwork for the understanding of colour vision and physiological optics. While some specifics have been updated, the basic premise still holds true today.
The natural primary colours
Another perspective on primary colours comes from the natural world rather than color theory. Red, green and blue correspond to the primary pigments found in nature:
- Chlorophyll – the green pigment in plants and algae that absorbs red and blue light.
- Carotenoids – various red, orange and yellow pigments produced by plants and bacteria.
- Anthocyanins – water-soluble pigments that appear red to blue depending on pH.
Since green chlorophyll is the most abundant and essential plant pigment on Earth, it could be considered a primary colour from this natural perspective. Chlorophyll appears green because it absorbs red and blue light more strongly than green light.
So in the context of visual biology, green is definitely a fundamental primary colour. The prevalence of green plants affirms this.
Green in colour models
How is green classified in popular colour models? Here’s a brief overview:
RYB (traditional pigment primaries)
– Red, yellow and blue are primary
– Green is secondary (mixture of yellow and blue)
RGB (light primaries)
– Red, green and blue are primary
– Green is a primary
CMY / CMYK (pigment primaries)
– Cyan, magenta, yellow are primary subtractive colours
– Green is not considered primary
HSV / HSL
– Hue, saturation and value/lightness
– Green is a primary hue
So green is considered a primary colour in some models, but not others. RGB and HSV treat it as a primary due to its part in colour vision, while CMY does not. RYB relies on the traditional pigment theory.
Four colour CMYK printing
In colour printing, black ink is commonly added to the primary CMY pigments to improve contrast and detail. This CMYK model is economical for full-colour printing.
But for very high quality printing, sometimes a fourth primary ink colour is added to expand the possible colour gamut. Occasionally this ink is green, which provides vivid greens, brighter yellows/oranges, and improved natural colour reproduction. Some companies may use a green ink instead of pure cyan for better print results.
So in commercial printing, green can be considered a fourth primary colour along with CMYK. This demonstrates again how green bridges secondary and primary status across different colour systems.
Green and colour vision deficiencies
There are various types of colour vision deficiencies which affect how people perceive colours:
- Protanopia – lack of red receptor cones
- Deuteranopia – lack of green receptor cones
- Tritanopia – lack of blue receptor cones
Those with deuteranopia are essentially colourblind to green light. This reinforces green’s role as a primary colour in human vision. The loss of green receptors significantly impairs normal colour perception.
Some researchers argue those with deuteranopia should adapt an alternative RGB colour model excluding green. This further illustrates the special status of green between secondary and primary.
Cultural variations
Perceived colour importance also has cultural factors. While red, yellow and blue are considered the traditional primary colours in Western culture, other cultures may emphasise different colours.
For example, in China red, yellow and black are seen as the most fundamental colours. Green is not always recognised as a distinct colour – the Chinese word qīng can refer to both green and blue.
So while green appears physiologically fundamental to human colour vision, cultural interpretations can differ. In Western colour theory green sits in an ambiguous space between primary and secondary.
Conclusion
While red, yellow and blue are considered the standard primary pigment colours, green occupies a unique position due to its role in human colour vision and additive light mixing.
Factors supporting green as a primary colour include:
- Helmholtz’s RBG colour theory based on retinal cells
- Its status as a primary additive light colour
- The prominence of green chlorophyll in nature
- Its appearance as a fourth primary ink in high-end printing
- Its absence causing the colour vision deficiency deuteranopia
So green exists somewhere between a primary and secondary colour depending on the context. In light and physiology it appears as a fundamental primary, while in pigment theory it is considered a blend of yellow and blue. This ambiguity makes green’s colour status fascinating and complex.