I've always been a big fan of the Heroes of Might and Magic series, and was fascinated by the variety of dragon creatures in Heroes 3. The above dragon designs are inspired by the game. From the upper left corner and going clockwise:
black (red) dragon--canine-like teeth towards the tip of the snout; plenty of spikes on each side of the face forming a "beard"
gold (green) dragon--closely resemble the black dragon but more gracefully built, with smaller "beard" and more elaborate horns
crystal dragon--dragons that live in underground caves most of the times so crytals start forming on their body
azure dragon--some of the largest dragons, with massive horns growing all over the face
rusty dragon--close relative to crystal dragon; they have the most developed "frill" on the back of the head