And there's also a violet dragon in the backstory. They might call it a shore dragon just as well. The heroine can assure those who hear the story that it is, indeed, violet in color. Or perhaps a blue-green-violet color, inconsistent from moment to moment.
Of course the characters would have to make the dragons' powers and abilities clearer, but considering it's a tale told to younger adventurers to educate them about dragons, that's easily slipped in. All the more in that these characters are legends in their own time (however much they hate it) because they are dragon slayers.
Of course, this is also the world where any kind of vaguely humanoid monster is a goblin, sometimes with unusual subtypes being given names -- or several names, with regional variations. It's not like folklore doesn't offer examples enough -- and folklore lets you worry less about trademark.