A relatively rare phenomenon in clouds known as iridescence can bring up unusual colours vividly or even a whole spectrum of colours simultaneously. These polar stratospheric clouds clouds (…) are formed of small water droplets of nearly uniform size. When the Sun is in the right position and, typically, hidden from direct view, these thin clouds can be seen significantly diffracting sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colours will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too angularly far from the Sun to exhibit striking colours.
(Image: Goran Strand)