here is a explaination from a friend of mine that you folks may or may not find useful. I rely on this person for expert advice.
True wheaten? Well, if you breed a BC that carries wheaten, then it's base is half wheaten, half birchen (with gold instead of silver). So, you can eventually breed the base out to pure Ewh (wheaten) and pure E^R (birchens for black coppers).
So crossing a E^R/eWh with another E^R/eWh will give you 25% eWh/eWh (pure wheatens), 25% E^R/E^R (pure birchen), and 50% mixed (E^R/eWh).
Telling the pure wheatens will be easier than identifying the BC's because birchen is dominant and they can look just the same as a homozygous birchen base.
I hope this help some ppl working with that color.