This is Why … The Colour Blue is as Rare as a Blue Moon

Now emerging from a long, grey winter in southern Ontario, our daily walks around the neighbourhood are becoming a technicolour experience, with flowers popping up and trees leafing out everywhere.

Daffodils dripping in their canary-colored finery, tulips blazing in fiery displays, pink canopies dancing overhead and every shade of green gleaming underfoot; spring is just beginning to show off. But where is all the blue, other than in the flawless skies? Even the fabled bluebells appearing at the top of this post are really more purple than blue …

Colourful flower stall in the streets of Dublin, Ireland with decidedly little blue on display (photo credit: Hannah Svensson)

Not just flowers and leaves, but fruits, vegetables, and berries all tend to appear in shades of reds, oranges, yellows, and greens. Yes, blueberries, but we’ll come back to them in a minute. Why does nature seem to discriminate against blue-hued design? For solid objects like flowers or berries, we see them as specific colours because they contain certain pigments – substances that absorb specific colours (wavelengths) of visible light to turn into energy, while reflecting the other colours to our eye.

The wavelengths of visible light, with violet around 450 nm increasing up to red in the 700 nm range. Ultraviolet is light that is lower in wavelength than we can see and infrared is light that is higher in wavelength than we can see.

For example, oranges, carrots, and pumpkins contain carotenes, which are pigments that reflect oranges and yellows but absorb the other colours. The new leaves emerging right now contain a lot of chlorophyll, a pigment that absorbs blues and reds, while reflecting green wavelengths.

In order to appear blue, the object would need to preferentially absorb all of the longer wavelengths of visible light and reflect the shorter wavelengths of blue and violet. But the shorter wavelengths of light are higher in energy, so these are the prime little packets of light to absorb. Generating complex pigment molecules is costly, so it makes sense for organisms to preferentially generate pigments that absorb the shorter wavelengths of light (blue/violet) for more bang for the buck, and reflect the less energetic longer wavelengths of red, orange and yellow to our eyes. This cost/benefit analysis of energy absorption translates into blue flowers being produced by less than 10% of the approximately 300,000 flowering plants in the world. It’s all about the energy balance.

The scarcity of blue in nature meant that the blue pigment known as ultramarine, made from the lapis lazuli stone, was more valuable than gold in medieval times and was primarily used for illustrating precious manuscripts such as this page from the Book of Kells.

But wait, what about blueberries? Couldn’t those monks have just smushed up some berries for their paints?

Blueberries ripening on a bush (C)Ian Lee

Actually, blueberries contain no blue pigment whatsoever! Instead, the skin of the berry contains a red pigment, but that skin is then covered with a waxy outer layer. Visible light scatters from the waxy surface, with the shorter blue wavelengths scattered more than longer reds and yellows, so the berries look blue. This is the same reason the sky or the ocean look blue – the shorter wavelengths get scattered towards our eyes more than the longer ones. You can see the underlying pinky red pigment in the skin here in the berries that are not yet covered in the waxy outer layer.

For the few plants that expend that extra energy to generate pigments that absorb the longer wavelengths and reflect blues and purples, there must be something in it for them. The conjecture is that these flowers preferentially attract pollinators who can see into the UV portion of the spectrum, just beyond the wavelengths that we can see. So while you’re out enjoying nature’s spring spectacle, take a few extra minutes with the blues and purples on display – like so many things in the natural world, their relatively rare colour can be explained by understanding a bit of chemistry and physics. Rather than spoiling their magic, I think that enhances it!