Due to his extreme state, it would be difficult to determine what he would have looked like if he had recovered. However, due to what coloring he had left and due to his fin type, it is likely he would have looked something like this:
He had a mostly black body with a greenish tinge and his fins were orange with some more green/blue hints to them. I don't know that he would have ended up that bright or not, and I think he would have been more green and less turquoise, but that's the closest I've found to his coloring. If fish like that are overlooked at the pet store, what chance to the average bettas in the back have? I'm glad I don't look at them.. I wouldn't be able to stand to go in there at all. I only see those because they keep them where I can't help but walk by.
This guy has some of his colors too. He would have been more black like the above, but the green on this guy would have replaced the green tinge to the guy above. Also the yellow in his fins was some combo between this guy and the guy above.. less vibrant than above but brighter than this guy:
His body may have been like this with the orange fins of the first, unless the green were to take over once he recovered.