Alright, so you've got your Red Dragon Disciples. They've got a tinge of Red Dragon blood in their bloodstream, and are powerfulish spontaneous arcane casters.
But... What about those with Green Dragon blood, or Copper Dragon blood?
Instead of the Red Dragon Disciple class, why not simply the Dragon Disciple class?
One would select a Dragon Type feat at first level in the PrC; Draconic Blood: Green, Gold, Copper, Red, etc.
From that, the other DrD abilities would be determined.