Excellent color vision, as with most primates, allows identification of ripe fruit from a distance. Conspicuous colors and markings, particularly on the face and tail, allow easier recognition of the species. The trait is, of course, for other monkeys, not for humans, although humans certainly use it. Even when only a small portion of the body is visible through the foliage, a monkey can tell who is who. Excitement level is indicated by tail position and coloration.
Long tails are certainly for balance and steering but they also indicate mood and status.
Heightened excitement level is easily recognized in skin color, which is enhanced by blood flow. In humans it is called a blush. Guenons have large, well-developed cheek pouches. All guenons are diurnal. Territorial defense is not left to the males, the whole troupe defends the boundaries, the females tending to be the fiercest in group conflicts.