They are both C/H. orientalis (chinese fire bellies). I have a breeding group and the females are almost twice the size of my males. If one were a Japanese (pyrrhogaster) then it would have a different patterned belly, not necessarily a different colour.
I agree that an unvaried diet may have contributed to the difference in colouration. All juveniles of this species start off with yellow bellies and turn red as they accumulate the products within their food items. Try feeding them plenty of bloodworm and brine shrimp in their diet and their bellied should redden more and look more even.
They look like a good pair though, good luck.