Well most of the Corps are takes on pre-existing concepts so it's not so bad. Yellow was always there (yellow impurity, Parallax). Blue is a splinter of Green. Red is taken from the 1980s Alan Moore stories. Purple has been around forever (Star Sapphire). Orange is from the Vega Sector right where they've been forbidden to enter for decades of continuity. So really Indigo is the only one out of scratch, right?