In order to locate the candidate facial regions, you need to model the skin color which requires
choosing an appropriate color space at first. Several color spaces have been utilized to label pixels
as skin including RGB, normalized RGB, HSV (or HSI), and YCrCb.
Although the RGB color space matches nicely with the fact that the human eye is strongly
perceptive to red, green, and blue primaries, unfortunately it and other similar color models
(such as CMY, CMYK etc.) are not well suited for describing colors in variant illumination
environments.