Not entirely true and is more of an "old wives' tale" regarding the yellow. In general Anery A have more yellow than charcoal (anery b) but that is definitely not always the case at all. The most surefire way to tell is the eyes. Charcoal eyes will be virtually solid black, whereas anery A will have a grey/silver iris with black pupil.
That's the best way to tell what your two are. And type a and type b are incompatible meaning that they're on separate locus. Breeding the two traits will give 100% hets for both genes.