Dragons in the Empyrean series come in different colors, and the color tells you a lot about a dragon's size, temperament, and standing. The exact ranking system is detailed across the books, but two colors dominate Violet's story.
Black dragons are the biggest and most dangerous. Tairn is one, and his size alone stops a fight before it starts. Riders bonded to black dragons carry serious weight in the Quadrant.
Andarna is a golden dragon, small enough that other cadets write her off as weak. That mistake is the whole point — her size hides an ability nobody saw coming.
Dragon tail shape is another identifying feature across the series; the books describe several distinct tail types. For exact names and rankings, the source books are the best reference.