Mandrill

The mandrills of West Africa are large-bodied, travel in small groups,
and are noted for the bright colors on the rump and face of the males.
The male mandrill's fearsome face is an extraordinary combination of
a brilliant blue muzzle and bright crimson snout. The female is duller
in color.

Large W African forest-living baboon 'Mandrillus sphinx', most
active on the ground. It has large canine teeth like the drill
'M. leucophaeus', to which it is closely related. The nose is
bright red and the cheeks striped with blue. There are red
callosities on the buttocks; the fur is brown, apart from a
yellow beard.

The mandrill, P. sphinx, a large baboon of western Africa,
has an enormous head, crested and bearded, and almost no forehead.
Another west ern African species is the drill, P. leucophaeus,
somewhat smaller than the mandrill, but similar in temperament.
The mandrill and drill are placed by some experts in their own genus,
Mandrillus.

According to Lauren R., the bright colors on the face of the male mandrill brighten when challenged!