The fact remains, if you're going to toss a dragon at the team to give them an idea of what dragons are truly capable of, you have a few options.

If they decide to wait till night-fall to go to the nearby place, send a black dragon adult at them. At night. They're possibly the stealthiest of the dragons, the evil one's, anyway.

If they go at any other time, give them a frozen hell with the white dragon.

I'd personally throw a green one at them, but that would wreak a bit of havoc on their equipment...