The Monster Manual has a bunch of suggestions for this. (It gives rather short shrift to white dragons, but I suppose that makes sense given that they don't even reach human-level Intelligence until ancient.) Black dragons get lizardfolk and plant monsters; blue dragons get elite human minions; green dragons get evil jungle creatures and corrupted/mind-controlled elves; red dragons get chaotic evil humanoids like orcs.

Beyond that... I love the idea of a white dragon using zombified minions, including frost giants, as long as you can handle the inevitable "Game of Thrones" jokes. In fact, undead minions in general would fit well for evil dragons. A black dragon could have armies of rotting swamp zombies, or packs of ghouls. A blue dragon might take over an ancient pyramid and seize control of the mummies within. Green dragons, with their love of subtlety and deception, would be a good fit for sneaky undead like wights and vampires. For red dragons, I'd probably go with a handful of powerful undead such as wraiths, death knights, or liches*, who would then have minions of their own.

[size=-2]*A death knight could easily be a willing servant; death knights are all about slaughter and destruction, and red dragons supply that in plenty. A lich might enter the dragon's service with the intent of subtly manipulating its "master," or it might be forced into servitude by a threat to its phylactery. In the latter case, the lich will be a dangerous and treacherous minion who might well provide covert aid to the PCs--but if you beat the dragon with the lich's help, watch out afterward.[/size]

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