The sky is usually the brightest part of a given scene, and often saturates – if the rest of the scene is normal brightness, parts of the sky will be too bright for the display to cope with – they appear white.

This can be made worse if you don’t gamma-correct your image. Monitors – CRT’s and LCD’s are non-linear displays: they perform a gamma-operation on the image you send them, so you don’t get the colour you ask for.