Cute toads! I'm afraid they're not hybrids though (well, I guess that's a good thing actually), but I'm impressed that you would at least suspect that! Most people assume Oriental fire-bellied toads are the only ones and forget that there are other species kept out there, so that's cool that you'd recognize the brown from another species
Though the brown coloration is characteristic of the European fire-bellied toad, captive orientals will sometimes exhibit this color. I'm not really sure why. The color doesn't go away from what I've observed, but it could possibly be due to inbreeding, stress, or perhaps it's a genetic adaptation to being kept in colder-than-preferred conditions. I have a brown and green blotched one myself, as well as an all-brown one. Another color oddity found in Oriental fire-bellied toads is a yellow belly instead of the usual reddish-orange, but this is due to a diet low in Beta Cerotene and can be reversed.
European fire-bellied toads are far less common in the United States, and beside having brownish-grey dorsal coloration, they also tend to have white flecks within the black on their bellies, often a slightly less vibrant orange (more of a yellow-orange than a red-orange), and (at least to me) they seem to have a slightly smaller head in relation to body size.
