From my experiences, female jack dempseys are nearly as colorful as male jack dempseys, although perhaps not quite as large or bright. I believe it is mostly the african cichlid species that have the far less colorful females.
Finsandfangs is correct about the gill plats but the males will have blue going all the way through there hole bodies (except the gill plats) when the females will have little blue if any on its body... They also can be vented but usually is uneeded do to the color differences...