A simple solution I had to this ran thusly - the color of a dragon is not a species, but an adaptation. Dragon eggs are "generic", as are dragon hatchlings (a departure from standard rules, I know). When they reach a certain age, the disperse from their parent's lair, and move some distance. They then develop characteristics to match the terrain in which they finally live - so, a dragon that winds up in tundra or otherwise cold region grows to become a white dragon.
I like this a lot.
My homebrew does something similar, though not so far to the "generic" as Umbran, but more "random"...well, at least split along the chromatic and metallic lines.
The preferred, though not always plausible method, of course, is mating with one of your own genus (red mates with red, silver mates with silver). These produce, also "of course", small clutches of the single color/metal "true" wyrms. These are generally quite small, however, only 2 or 3 eggs. Depending on the genus, the eggs may or may not be cared for/guarded and the offspring may or may not be reared to a certain point. In the case of coppers and silvers, large groupings of extended family units, some of several generations, may lair in close proximity if not within a single lair.
Any metallic can mate with any other metallic resulting in clutches somewhat larger (maybe up to 5 or 7) and produce a mixed bag of offspring. They can be of any metallic type, not just the parentage and not really discernible from the shell (perhaps a slight hue or sheen offers a hint). The clutches are laid (somewhere presumably safe if not impossible to reach) and [usually] left. As with many reptiles, parenting isn't a huge imperative. The hatchlings are born with nearly all faculties and ability needed to find their way across the world to a terrain of their liking/dna and age/grow normally. If they find/encounter others of their kind, as coppers and silvers in particular seem hard-wired to seek out, they may increase their knowledge/abilities/receive formal "upbringings."
Any chromatic can mate with any other chromatic. The resulting broods are significantly larger (between one and two dozen!), similarly, mixed bags of any possible chromatic AND a variety of mutations/sub-dragon offshoots. Linnorms, hydra, wyverns and behirs are the most common results. When they're born, those that survive their siblings, are similarly genetically wired to seek out their preferred territories to fight their way into adulthood.
Dragons that mate with other draconic-but-not-"true"-dragon creatures (nearly universally chromatic) invariably have offspring of draconic mutations of the non-dragon creature. This is also the case in the near unthinkable mating of a chromatic with a metallic (producing a mutated sub-dragon creature). A black dragon and a behir, for instance, will result in a black scaled/skinned acid-breathing behir. A blue dragon and a wyvern might end up with a blue wyvern that may or may not have lightning breath but, perhaps, is resistant to lightning damage. Mating with a linnorm might result in a linnorm that has wings capable of flight. The exception here are the brass who are believed to occasionally mate with dragonnes (dralions, in my world) that bare unaltered dragonne offspring, exclusively.
So, not pure/clean slate/generic offspring, but a variety of possibilities for outcomes of crossbreeds.
