On Pern, humans form a telepathic bond with dragons which is known as Impression. When a dragon's clutch begins to hatch, prospective bonds--candidates--are brought onto the Hatching Sands. As each dragon hatches, it seeks out the best match for it from among the available candidates. The weyrling dragon and rider train to fight Thread until they are adults and join the rest of the Weyr in fighting Thread.
Thread is an extremely dangerous, voracious but unthinking lifeform which appears as long, silvery strands reminiscent of its namesake. It comes from a nearby planet and periodically passes into Pern's atmosphere, where it falls to the ground and starts devouring anything it comes across. Dragonfire is one of the few things that can destroy it in the air, which is why the dragons are so important.
There are five different colored dragons, each with their own specialties in Threadfighting. From largest to smallest, (and rarest to most common) they are:
Gold: These are the queens of the Weyr and the only female dragons that can breed. They are by far the rarest dragons, and significantly larger than their siblings, even as an egg. You'll definitely know if there's a queen egg on the sands. They're too valuable to risk in Threadfall, so they fly low to the ground and mop up any stray strands that are missed by the other dragons.
Bronze: Bronzes are the largest of the male dragons and most likely to breed with the queen. They have the most stamina and the largest flame, so they are very good at searing Thread--that large size also makes a larger target, though, so they are more likely to be hit by Thread as well.
Brown: Smaller than their bronze brothers but still large enough that they can catch the occasional queen, browns are the steadiest and probably most average of the dragons in fighting Thread. They typically act as lieutenants to the bronzeriders.
Blue: The smallest of the male dragons, they're a lot quicker in the air than bronzes and browns, but still able to last a good while. They're most accurate with their flames and miss Thread less often.
Green: The smallest and by far most common color, the female greens make up nearly half of the dragons in a clutch. They often rise to mate, and though they're chased by any male dragon, the firestone they chew to produce flame renders them infertile. They are very speedy and agile in the air, catching strands other dragons can't, sometimes two at once! Fast as they are, they're also fragile and tend to take more damage when hit, and their frail endurance means that they only fight part of a Threadfall, switching out for fresh greens.