How Skin Coloration arises

Sometimes it is good to recall the fundamentals: he skin consists of a number of layers with distinct function and distinct optical properties. 

White light shone onto the skin penetrates superficial skin layers and whilst some of it is absorbed, much is remitted back and can be registered by a camera.

The key to the interpretation of image colors in terms of the underlying histological parameters is a model of tissue coloration which provides a cross-reference between the colour and the histology. 

Here is a model discussed ( Weblink)

This model is constructed by computing the spectral composition of light remitted from the skin given parameters specifying its structure and optical properties. This step needs to be carried out only once. As the mapping between the colours and the parameters is unique for the skin, each colour corresponds to one specific set of histological parameters.

The model of normal skin coloration is representative of all the normal skins, irrespective of racial origin, age or gender. The structure remains the same, and the only differences are in the magnitudes of the parameters.

Abnormal skin coloration

The model above has been constructed for skin which has a normal structure. Skin coloration associated with abnormal conditions does not necessarily have to conform to this model.