In 2015, a YouGov survey conducted across several countries found blue to be the world’s favourite colour. Yet for such a popular colour, true blue pigments are incredibly rare in nature.
If you’re familiar with the sight of dragonflies flying around on a summer’s day, or the vibrant plumage of a peacock, this may seem strange. However, these iridescent shades that we all know are caused not by pigments, but light manipulation.
The blue colour relies on the presence of a transparent layer with a black surface beneath it. Within this transparent surface are many tiny ridges. The distance between these ridges has evolved so that a single wavelength of blue light spans across two of them. Since this isn’t the case for the other wavelengths, they are absorbed by the black surface beneath while the blue light is reflected away from the dragonfly. This results in the iridescent, shimmering blue colour found on dragonflies, peacocks, and several other species.
Many species use this method because true blue pigments are costly to produce. To appear blue and absorb all other visible light wavelengths blue, a pigment must absorb low-energy red wavelengths. This requires them to be larger and more resource-heavy.
Evolution means that traits only persist through generations if they do not harm the chances of survival, and in most cases, the cost of producing a blue pigment outweighs the benefits associated with it.
There are some exceptions to this. A few butterfly species produce a true blue pigment, such as the obrina olivewing. While there is uncertainty over its exact purpose, the mere fact that this trait has persisted across generations indicates that it plays some important role.
Plants may use blue flowers for the important role of attracting pollinators. They too use a pigment for this, but it’s not a true blue one. These pigments, found in blueberries, grapes, and cornflowers, change colour depending on their surroundings. Based on how acidic the surroundings are, the colour may be black, red, purple, or blue.
This blue colour appears when in a neutral solution, such as pure water. At school you may have boiled red cabbage and used the liquid as a pH indicator; if so, you’ve seen this in action. The colour-changing nature of this pigment means that animals can’t ingest and use it for their own pigmentation, as flamingos do with their iconic pink colouring.
The rarity of blue pigment in the natural world has had consequences for how we’ve used the colour and the symbolism associated with it. For much of history, the colour was relatively hard to obtain to use in paints and dyes. This resulted in blue being associated with royalty as the average person could simply not afford it. Synthetic dyes have since increased accessibility to the colour and now it can be seen everywhere in the man-made world. Yet in nature, the rare blue glimmer of a darting dragonfly caused by a simple trick of the light is still just as eye-catching as ever.
