From D&D Wiki
What happens if two dragons of different colors mate? I don't think it would make a half dragon/dragon I think it would be a new dragon all together. what do you think?
I would assume it is something similar to when Labradors breed. I don't know how much biology you know, or how much you're willing to learn, but a Punnett Square could help answer this. In my presumed example world, color is controlled by a few different alleles(R and Y, named to match the tables shown on the wikipedia page), as well as "B" and "C". We can assume that allele "B" controls chromatic/metallic in dragons. We figure "BB" or "Bb" would be Chromatic (as they are more common in my example world) and "bb" would be metallic (as they are less common). After some dihybrid crosses using Punnett squares we can get something like this: black(RRYY, RrYy), blue(rrYY, rrYy), green(RRyy, Rryy), red(rryy). Metallic dragons then look like this: brass(RRYY, RrYy), bronze(rrYY, rrYy), copper(RRyy, Rryy), gold(rryy). You may have noticed I left out white and silver. A fourth allele is present "C". "C" controls pigmentation. When "CC" or "Cc" treat as above (these are normal dragons), but when "cc" we are to assume the dragon formed is either white or silver (as determined by the B allele, these dragons lack pigment). This system creates a full spectrum of dragon colors. Some obvious problems occur within this system. Namely, because each dragon lives in very different environments, it's unlikely that 2 blue dragons would raise a white dragon, or a red one. However, using this system it appears that this certainly happens. What I'm going to say is "Fearing for the sake of their color-matching offspring, mother dragons often kill offspring that do no match, and feed them to their young." Of course, the first rule of fantasy gameplay is don't bring real science into it, so you're more than welcome to say "a magic blending occurs, allowing a dragon to possess both traits of the mother and father. This rarely happens, but when it does, this dragon is always very powerful."