I have both strains from Jorg in culture. An important thing to know is that both black AND yellow underwings were obviously mixed in his roaches. So, or both are from the same species, and it's just an inner species variation, or they can hybridize and in that case, botch of my colonies are just bullshit. By security, I've kept all black hindwings adults in a tank, and all yellow hindwings in an other one. I'm waiting for their babies to get adult so I'll know it has a chance to be two different species or not.
Originally, all those Therea were sold as Therea petiveriana. The strain was obviously ID by Fabian Deck in Belgium who compared them with other Therea in a museum. I personnaly know him, and last time I saw him we discussed about the possibility it was not petiveriana. Indeed, this species was the more close to what we have in captivity, but he didn't made any wings comparison or anything, he was very quick to ID them and confess he might be wrong, but there were no other Therea in stock that looked like this.
I had a very quick talk with Ingo Fritzsche (who described Therea bernhardti) when I met him last year and he clearly told me the other species, with yellow underwings, was maybe from the same species. No one knows at the moment, but we shouldn't be too enthusiastic about this "new species". If my colony occur to be 100% yellow hindwing, I'll send him some samples so he can confirm me if it's new or not.
Note: what we have in culture is Therea bernhardti, not petiveriana. True petiveriana has hindwings longer than berhardti. When the ID mistake was spotted, Ingo described the new species but the wrong name was already very popular in the hobby. The two strains, yellow and black hindwings, are very short winged. In both case, they are definitely not Therea petiveriana