All about the dragons: Red, purple, blue, green and black.
Red: Production (Resources, construction, research)
Purple: Infantry (Defense)
Blue: Cavalry (Attack)
Green: Archer (Hp)
Black: Siege
To enhance dragon skills you need dragon glasses. To evolve/unlock your dragons, you need soul stones. Both can be acquired from certain level barbarian camps.
If you have any more questions, feel free to drop a comment below.