For my Dragonlance send-up, tentatively named Grimdark Dragonthing, I decided dragons were created as living weapons by the otherwordly, all-conquering serpent folk (think the kaiju in Pacific Rim). This is why they were color-coded for convenience.
While dragons were created initially as dumb beasts, in time (when their empire collapsed) some serpent folk downloaded their extraordinary, magic-adept intelligence into dragon bodies built to support speech and sorcery. This is why some dragons speak and cast spells, and some don't.
Of course, as their dominion crumbled and survivors grasped at the last straws of power, a cadre of particularly crafty serpent folk sorcerer-scientists created (or maybe just unearthed) the ultimate control mechanism for their biological flying tanks, including those piloted by the consciousness of their rogue comrades; the orbs of dragonkind.
The nascent human empires beat back the serpent folk and their dragons, at the cost of countless lives. Surviving dragons sought refuge in the desolated wilderness, typically in the terrains and climates they were specifically designed to operate on. But every now and then the orbs resurface at the hands of some would-be conqueror or doomsday cultist.