Red dragons are lightly built, with narrow wedge-shaped heads, short necks, and short tails. They have four short horns that extend out and back from the back of the head. Color ranges from brilliant crimson to a dull dried-blood tone. Eye color varies from shades of reds through yellow and even green (normal vertical slit-pupil eyes); older dragons often have eyes that are either solid black or red (usually either much brighter or much darker than the dragon's scale color).
Purple dragons are also relatively lightly built, and somewhat more cat-like than other dragon types. Their heads are shark-like (or nether-drake-like for the WoW players among us), accented with a pair of stubby ridges running from the tip of the nose to just a bit behind and above the eyes. A purple dragon's shoulders are decorated with a cluster of spikes (front shoulders only - the hips are free of any obstructions that might interfere with the wing membrane.) Coloration is almost never a single solid shade; typically a purple dragon has a lighter colored underside, darker top, and often mottled variations within those general expanses. Eye color is usually green, blue, black, or somewhere inbetween.
Blue dragons are hulking beasts - imagine a quadrapedal tyrannosaurus, and you won't be far off. While they don't have horns per se, the larger scale plates on their backs are ridged, and jut out a bit towards the rear edge, giving them a distinctive sawblade look. Coloration is usually medium to dark blue with wing membrane being only slightly lighter than the main body. Eyes are almost always red or black, though purple isn't unheard of.
Turquoise dragons are just as small and (comparatively) frail-looking as reds. Their heads have short muzzles, and two to six fins, usually along the sides of the head, but occasionally over the eyes, under the cheekbones, or forming a cowl around the neck. Sometimes these fins extend down the sides of the neck to join into the wings - which, in turn, often connect to fins down the sides of the tail. Color is almost always a bright blue-green on the scales, with darker tones on the wing membranes and fins. Eyes are red, yellow, black, or sometimes green.
Green dragons have a very serpentine build and head shape (as befits their usual combat style of grab, constrict, and swallow whole). Some have a pair of narrow horns sweeping back from the head, but aside from this they're more or less undecorated. Coloration varies from lighter (often near-white) on the belly and wings, to dark green diamond patterns on the back and legs. Eye color varies widely, and many green dragons have eyes of two different colors.
Yellow dragons have more variation between individuals than most other types of dragons. One may be long and thin, the next squat and angular. The only truly constant feature is a frazzled-looking frill of asymetrical spikes behind the head; often extending all the way down the cheekbones on either side. Most then have some other patches of spikes, as well. One yellow dragon might have a patch of spikes on its left front shoulder and another patch halfway down its tail, while another might sport a long row of spines marching down the neck, across the back, and ending in a rough spiral on the right hip. Eye color varies, much like green dragons, though the most common eye color reported by adventurers is "What, you think I'm foolish enough to look into the eyes of a yellow dragon? No thanks; I prefer to keep my sanity."
Black dragons are fairly heavily built, with a smooth, rounded, form. Their scales are an iridescent black, and their eyes glow with all the colors of the rainbow. Further racial description is largely meaningless, as black dragons are also natural shapeshifters, and tend to either customize their own form... or barely use it at all.