Listen, if playing D&D has taught all of us anything is that there is
nothing too stupid to fight.
Pink dragons, farting tentacled discs, strangely sticky treasure chests, tree stumps with squirrels on them that eat you, living transparent cubes, singing mushrooms that taste delicious, evil vampiric demon-worshiping manta rays...
As for dragon cross-breeds, I think the idea is pretty fascinating! You could go in a lot of different directions. IMC, usually environment and temperament lead to a kind of sexual selection amongst the dragons. Mostly this is just to keep my OCD in check because if I decided dragons could cross-breed,
I would frickin' make a matrix, and there goes a week of my life.
So reds and blacks stay away from each other mostly because reds love fire and blacks love swamps, and if the two were every to overlap in territory, we'd probably have a lot of toxic steam and then whichever dragon is more powerful would be left standing (probably the Red) and the other one would be dead.
...because if I started thinking about a "maroon" dragon, I'd start thinking way too hard about make-believe.
I've got enough dragons. Between the 5 chromatics (white, black, green, blue, red), the 5 metallics (brass, copper, bronze, silver, gold), the 5 gemstone dragons (crystal, topaz, emerald, sapphire, amethyst), and the 5 "precious stone" dragons (pearl, jade, amber, jacinth, moonstone), I've got enough to have a big dragon fight at each PC level and still have dozens left over! A maroon dragon isn't going to fill a gameplay niche I need.