Cloud cover is made up of various patches of cloud, each with a different density. As the plane ascends it passes up and through these different patches. 

The sunlight strikes the cloud cover from behind. Each cloud’s vapour takes on this sunlight to illuminate a specific hue of white, depending on it’s density.

Thinner clouds conduct into a bright yellow white, whilst denser patches drown out the sun’s rays, resulting in a duller grey tone.

As the plane hurtles through the cloud cover, this iPhone video captures the rapidly changing hues as they fill the plane’s window frame.