To understand colour saturation it is necessary to consider white light. White light, daylight in everyday terms, consists of a mixture of every colour of light the visible spectrum - all the colours of the rainbow, or those seen on the colour wheel. Indeed, the colours seen in a rainbow have been separated into their respective colours (or wavelengths) from white light, by the presence of droplets of water in the atmosphere. A rainbow is therefore revealing the components of white daylight which are ever present but not normally seen.
When an object is illuminated by white light, and its surface reflects back to an observer's eyes all the wavelengths of the light falling on it, the reflected light remains white. Although not all the light is reflected, the various constituent wavelengths are reflected to more or less the same extent. However, if the eye perceives a blue object, we know that all the wavelengths other than blue are being absorbed by the object's surface, and only blue light is being reflected. When an object appears black, almost all the incident light is being absorbed, and very little is reflected.
Colour saturation is a term used to describe how much of a particular colour or wavelength is present or reflected. It is also a measure of the purity of a colour - ie are elements of other colours mixed with the particular hue. If, for example, an object absorbs every color except blue, then its colour will be perceived as a highly saturated blue. However, if some of the blue wavelengths are absorbed with all the other colours, the object's colour will be perceived as less a less saturated blue. When all the colours are absorbed to a large extent, the reflected light approaches black and may be perceived as largely grey perhaps with a tint on one colour. Under such circumstances, an object may be described as having a less saturated colour. As the percentages of greys and blacks in an image increase, photographers therefore describe images as being less saturated. As the percentages of greys and blacks in an image become greater, so the image is regarded as more highly saturated.
It should also be said that saturation is not the same as brightness. Colour saturation is a measure of colour purity whereas brightness is determined by the amount of white mixed with the colour. A fully saturated colour is one of a particular wavelength uncontaminated by other elements. Red, blue and yellow are primary colours and therefore highly saturated when seen in their purest forms.
The optimum colour saturation for a particular image depends in part upon the subject. In the case of landscapes, photographers often choose to increase colour saturation. Human beings like to see deep blue skies and very green grass, but the effect can easily be overdone and become unreal. In portraiture, lower colour saturation often produces a more attractive or flattering result. Photographers who use film may choose the type they use partly on the colour saturation it produces. Some films are advertised as "highly saturated" or perhaps "suitable for portraiture". Digital photographers have the luxury of being able to choose the colour saturation they require, and can adjust saturation when processing images after they have been captured. However, care must be taken when undertaking such adjustments. It may not be appropriate to adjust the overall saturation of an image. Just a few colours, or perhaps just a single colour, my need a change of saturation. In this situation the overall image may be unacceptably degraded by simply sliding the saturation control in Photoshop. The job may be better done by creating a new layer and then masking in only those areas of an image where saturation change is beneficial.