Green and Gold Dragons are level 7 creatures of Rampart town. They are recruited from the Dragon Cliffs.
Green Dragons can be magically damaged by 4th level spells like Meteor Shower, Chain Lightning and Armageddon.
Gold Dragons are more resilient and can only be damaged by Implosion as it is the only 5th level direct damage spell, save for Orb of Vulnerability to counter spell resistance of any creature.
Indirectly damaging Green or Gold Dragons via magic is possible by either buffing the own groups with Slayer (which yields +8 attack against giants) or other buff spells or summoning a creature with any of Summon Air Elemental, Summon Water Elemental, Summon Fire Elemental, Summon Earth Elemental.
While they hit a bit harder and fly a bit faster than Black Dragons, they have 20% less health and lack the signature full magical immunity, making them populary targeted and dispatched by implosion, while having pretty much the same cost. Placing Hydras or any living Dragons in the Skeleton Transformer will give you Bone Dragons instead of regular Skeletons.