Here's what's happening:
You can find the original right here.
When the sun is positioned at precisely the correct angle behind clouds water droplets in them diffract light away, creating an intense streaking effect. The colouring is, like a rainbow, caused by the differing wavelengths of the light – different wavelengths are diffracted to different degrees, altering the angle of diffraction and consequently the colour of the light as it is perceived. In this image cloud iridescence is accompanied by a sharply coloured ranbow.