Neither are getting to fight Usman after winning this fight. Masvidal will never fight Usman again. Colby could fight Usman again, but he's gonna have to beat another one or two guys at least after Masvidal in order to get there. Plus, Colby is already talking about maybe fighting Poirier or even moving up to middleweight - I think those are way more likely for him than fighting Usman again.
But yeah, no way Masvidal fights Usman again, regardless of how this fight with Colby goes. It was clear from their previous two fights that he has nothing for Usman, especially since Usman knocked him out cold. Win or lose, Masvidal likely fights Diaz again, maybe even Conor, or some other money fight. Maybe he'll finally fight Leon after Usman beats Leon again. Regardless, no way Masvidal fights for the belt against Usman ever again.