Saturation in images

The saturation of a colour is never constant but depends on the surroundings and light. When a bold saturated colour is used with a film or a painting etc it is generally specifically done for a reason for the audience to interpret.

Saturation is also referred to as the purity of colour. Saturation is defined by light intensity and how much it is distributed across the different wavelengths. So the most pure colours are achieved by using just one wavelength at a high intensity such as a laser point.

http://designingfortheweb.co.uk/book/part4/part4_chapter17.php
This website clearly shows the different effects of saturation using photographic help. They clearly show how an image can be made more subtle, more bold, aswell as how it can improve or ruin an image by making specific choices about the saturation.