We believe that people should never breed just for color/money but for quality/temperment. But if you have two dogs of the same color that have breeding quality/temperment and the pups wouldn't be inbred, then go for it. We think that people breed a blue to a blue to produce more blues and the same for reds.. they'll breed a red to a red to produce more reds. Their shouldn't be any problem with breeding a red to a blue.. it would just produce a more diverse colored litter of pups.
The Colby's prefered to not breed red noses... not sure why though... and back in that time their was no such thing as a blue nose or a blue APBT.
And on the subject of the OFRN (Old Family Red Nose) bloodline... a pure OFRN is completely red from their coat, to their nose, to their toe nails and if we remember right even their whiskers are red! So if a person says they have a pure OFRN and it isn't completely red, then they are mistaken.
As for 'rarity' and the APBT breed : One color is not any more rare than another color, possibly more unique but never more rare. What we personally believe to be somewhat more rare would be a pure APBT of one of the eldest original strains/bloodlines such as a pure Colby, Carver, Tudor, Corvino, Clark, etc. But their are so many breeders keeping the pure form of these bloodlines alive that they really aren't too rare. We personally also believe that these lines should cost more than a RE or Gotti because those are the original bloodlines and without them we wouldn't have the APBTs that we own or breed today, but for some reason that's just not the way it is.