Blue is the color of Rayleigh scattering in Earth’s atmosphere. The light coming from our sun is broad spectrum with violets, blues, greens, orange and red colors and is scattered when it enters the Earth’s atmosphere. But blue light is around 450 nanometers and is more strongly scattered than the other colors giving us the blue color of the sky.
Blue is a historically difficult color to make pigments of. The earliest documented blue dyes for fabrics were made from plants, notably the woad plant in Europe, the indigofera plant species in Africa and much of Asia. For blue paints pigments, minerals like lapis lazuli or azurite were the source.
Blue also tends to get our attention in nature aside from the sky or ocean. The blue quandong fruit for example or the Lactarius indigo mushroom are dramatic examples of plants and fungi that are blue. Jay birds among other blue colored birds, fish, frogs, lizards and insects are all dramatic examples of blue animals and every one of them grabs our attention. One of these days, I’ll get Robert to deliver his color talk when I can record it, so it can be shared, but until then, know that the human visual system is strongly attuned to blue and the “neurology of blue” is a fascinating topic.
The image above was made with my iPhone pointed up at a blue sky without a cloud in it and was the most intensely blue sky I’ve seen in a while as seen by the color matrix below, reduced to 8-bits. Some of the lighter colors of blue comes from noise in the camera from jpg conversion and compression, but there is also a gentle gradient of color to lighter towards the bottom of the image as light was more scattered through more atmosphere closer to the horizon.