If we analyze the face region:

We can discover the skin color distribution clusters in a small area of the chromatic color space:

We have further found that distributions of skin colors of different people are clustered in chromatic color space. Although skin colors of different people appear to vary over a wide range, they differ much less in color than in brightness. In other words, skin colors of different people are very close but they differ mainly in intensities. The following Figure shows a skin color distribution of forty people with different skin colors in the chromatic color space. The distribution was obtained by analyzing faces of different races, including Asian, African American, and Caucasian.

By closely investigating the face color cluster, we have discovered that the distribution has a regular shape. A close view of skin color distributions is shown in the following. Figure (a) and (b) are color distributions of a face under different lighting conditions and (c) is the color distribution of two person's faces.

It is obvious that the human face colors of different people under different lighting conditions in the chromatic color space have similar Gaussian distributions. Therefore, a face color distribution can be represented by a Gaussian model.

Since the model only has six parameters, it is easy to estimate and adapt it to different people and environments.