Why do we look darker when we sweat?

Though i have seen this many times this is the first time I am giving thought about why this happens

in short:Why does anything appear dark at all?Because the reflect less light.Even in this case it is the same.(How?)

Normally light just reflects off a surface into air then reaches our eyes(simple diffused reflection).Here water forms a thin layer over the surface,hence allowing light within a particular angle [called critical angle(48.59 degrees for water)] to make it out of water layer (Total Internal Reflection).This ensures that light beyond this particular angle is blocked =>lesser reflected light.

in detail:  http://seblagarde.wordpress.com/2013/03/19/water-drop-3a-physically-based-wet-surfaces