Note how the red colors really stand out in the face of this yelling man. When a person of any skin color opens his or her mouth to yell, the redness in the individual's lips, gums and mouths become very evident. This man's skin is redder in color too, having received a rush of oxygenated blood due to elevated testosterone levels.
A friendly smile, on the other hand, puts the emphasis on white teeth as opposed to redness in the mouth. This and the "angry red" signaling are particularly evident in males.
Wiedemann explained, "Males in a variety of species, be it human or nonhuman, are by nature programmed to engage in competitions with other males and compete for mates, territory, food, status etc. Therefore, being sensitive to the signaling meaning of red coloration as a possible sign of dominance may be helpful for males to assess an opponent's physical abilities and then reflect on whether it is worth engaging in a fight (and hopefully win) or not (thus avoiding injuries, for example)."
"In contrast," she continued, "females are biologically less competitive, and as a consequence, the association between dominance and red is less important for survival among females than it is for males."
Hawking: Our Aggression Will Destroy Humanity