”The sun had recently set, and the west heaven was hung with rosy cloud, which seemed permanent, yet slowly changed. To watch it was like looking at some grand feat of stagery from a darkened auditorium.”
Thomas Hardy, The Mayor of Casterbridge – The Life and Death of a Man of Character, 1902, Chapter 1
The scattering of sunlight is responsible for the wonderful colors of the sky. But it is also responsible for the equally beautiful colors of clouds. However, the individual droplets of water which form a cloud are much bigger than the molecules present in the air. And unlike the atmosphere’s scattering effect, the scattering effect of a cloud scatters all wavelengths. This explains why clouds usually appear white to us.
The thicker a cloud is, the more sunlight is blocked, making the cloud appear grey. In the absence of direct sunlight, clouds can reflect the color of the sky and may, for example, appear bluish. The amazing colors of clouds at twilight are caused by both the scattering effect of the atmosphere and of the clouds themselves, giving us clouds in a multitude of yellow, orange, and red hues.