In reality I too think it is an ambiguous notation. Since Copper coins tone from so called Red to Brown gradually. Just where in that realm of graduations of color could someone say RB or BR or RRB or anything. True when exactly inbetween R and B, someone could say RB but even then, is it RB or BR?
Just as it has been done with grades from 1 to 70, I suppose eventually someone will come up with a similar numbering system for RB. Such as R5B in a 10 point system that would mean half way to B from R. Until then, RB is just to vague.