I think the explanation for the black Ripper is simple: Given that by then the white suit became his trademark, he probably had the natural Ripper refinned to black - a natural Ripper wouldn't look good on a white suit under all that lighting in an arena show, simple as that. Before he went all chubby, Greg was very vain about his angelic looks. He was the only reason why some guys could drag their girlfriends to ELP concerts for two hours of music mostly alienating to female ears, the soppy Lake ballads excepted!
My understanding is that "ebony"-colored Rippers always came with an ebony fretboard and the only way to get a maple neck was buying a natural one, but that might have been different when they came out. And Lake had his eyes on a (then) new Gibson bass so early he even visited the factory and saw those immensely heavy singlecut Ripper prototypes that were still sans a pickguard and therefore unrouted.