Re: how lip color relates to skin color? Is there a specific ratio between two?

Hi Jigna,

  You pose an interesting question, and one which I believe deserves both a
formal and an informal reply.  First, it's important to separate the
concepts of color and intensity: color is a matter of the wavelengths of
light reflected, whereas intensity is a measure of how much light is
reflected.  Two major pigments determine the color of human skin, hair and
irides (the iris of the eye): phaeomelanin and eumelanin.  Phaeomelanin in
low concentrations appears yellow, while in higher concentrations it
appears reddish.  Eumelanin is black.  The ratio of these pigments
determines color, whereas the amount determines intensity.

  Using hair color as an example, absence of both pigments produces white
hair (as in albinos or the elderly).  In the absence of eumelanin, low
levels of phaeomelanin produce blonde hair, and high levels produce red
hair.  Low levels of eumelanin and phaeomelanin produce brown hair, and
high levels of eumelanin produce black hair. Skin color works similarly. 
Eye color is different because the base color of the iris is blue, so low
levels of phaeomelanin appear green, high levels appear amber, low levels
of eumelanin appear brown and high levels of eumelanin appear dark brown.   

  As I am not aware of published data on the pigmentation intensity ratio
of skin and lips, I simply did an informal survey in our lab.  Disregarding
color, it is clear that dramatic variation exists in the intensity ratio of
skin to lip, probably reflecting both genetic and environmental influences.
 The environmental influences include tanning and lipstick, and I assume
that there are genetic components as well because of what we already know
of the genetics of melanin.  So, informally I'd say that an algorithm based
on pigmentation intensity alone won't be particularly robust, particularly
across ethnicities.  You might do better with a geometric triangulation
from the eyes or ears, or by looking for bilateral symmetry, but biometrics
isn't my area of expertise.

  Chris

Sturm et al provide an excellent review of the genetics and biochemistry of
the melanin system in Gene 2001 Oct 17;277(1-2):49-62