It's at clvl 7.
And if you turn grey, you don't have a breath weapon but you have a more powerful tail and gain more natural armor, compared with other draconians, as you level up. Otherwise, this one seems rather inferior to the rest.
EDIT: Red, white and black draconains also gain a resistance at clvl18 (fire, cold and electricity, respectively). Green draconians gain poison resistance at clvl7.