Also, I think that a fight with a dragon should not be just a "Fight with a dragon", but it should be an Event. Dragons are the masters of their element, and are going to fight on their terms; half the battle should be contending with the battlefield as the dragon plays cat to their mice.
You're not just fighting a green or black in a 30x30 room. You're fighting it on a lake with lots of logs strewn about, so the dragon can play "Jaws", hitting and running, while the PCs jump across fallen logs.
A battle with a White is going to be on a mountain pass, where the ledges are icy and the thing can take to the air, bullrush people off of the ledges, or cause avalanches/rockslides.
A blue might attack during a sandstorm, using the blinding sand as concealment to circle and blast the party as they fight against the strong, stinging winds.
A battle with a Red in a forest, where the Red knocks over trees, boxing the PCs in with the difficult terrain, and sets the forest itself on fire. The PCs might have to flee or else getting out alive once the dragon is dead is a problem! Or the Dragon fights in a cave where it targets the rock beneath teh PCs, superheating it until they can't walk on it without taking damage.