Draconic sorcerers have dragon heritage mixed with some non-dragon ancestor. So is a red dragon got busy with a blue Dragonborn, that’s a way to arrive at the situation you describe.
Dragons actually would never mix blood.
Thy are driven to choose only partner of their own heritage. Hence why you never encounter dragon of strange colors. Albeit in lore there are rares types of dragons. Like Song Dragon, Fang Dragon (Grey Dragon starting from 4th edition), and other. Still none comes from mixed blood.
As of OP. Roleplay-wise - yes. It would be proper to choose dragon of the same heritage. Alas 5e doing very poor job on restricting such situation, even if that means clearly broken immersion. You can have CE Paladins of Redemption... you can have chaotic monks, lawful barbarian. And you can have abominations like Gold Dragonborn, with Blue dragon bloodline for sorcery.
