Although aquamarine is also the name of a gemstone so I think it's better off for a gem dragon. For me blue+green=teal
Now on to breath weapons.
The traditional ones (acid, cold, electricity, fire, force, light, and sonic) have been used ad nauseam. The various poison and disease effects, along with the sand dragon's flaywind effect, and the faerie dragon's euphoria effect, are innovative. There's also a breath based on a spell, the prismatic dragon's prismatic spray breath weapon. Let's come up with some more, like color spray (for my opal dragon, which appropriately is multi-colored). What about an anti-magic breath weapon?