I don't think it's a separate gene at all. I think it's just a different "line"...

The graphite ivory looks cool and all but what are the chances of producing that? Very slim I think because I believe that has something else going on with it. Like a hidden gene or subtle morph like the Lori, Special, Russo, etc. something could have been present in one or both of those original parents that caused the graphite. Which means, as time goes on, that "something" will be lost with the generations and you'll never know if you have a shot at making something as cool as the graphite, or of you just have a yellow belly with a different name..