Males are dark blue or nearly black, with a dark face and dark stigmas. Females and young males have a black stripe down the abdomen and a pale pattern on the side of the thorax said by various authorities to resemble a wolf's head or a parrot's head. To me it looks nothing like those, maybe a rhinoceros head makes as much sense as anything.
This thorax pattern is shared by Great Blue Skimmer and Bar-winged Skimmer; Great Blue has a whitish face and Bar-winged has light edges to the sides of the face. The overall coloration of these species may be brighter as well.