Hifumi the witch was once Kuro's mother.
Volume five's first chapter tells the story of three women who would become the witch Kuro is hunting for. The third woman had a daughter who looks exactly like pre-curse Kuro before she cut her hair in volume four's first chapter which told the story on how Kuro and Sen met. When/If the two meet up again the truth will be learned and it will be bittersweet. Gray is the result of mixing white into black.
One of the first few chapters takes place after Kuro decides to take Nijuku and Sanju out of the lab and the pair show off their color absorption abilities which would soon result to Nijuku getting her black ears and tail. Kuro explains the reason this happened is because no matter what color you mix into black it stays black but that is actually not true for anyone who has had some experience with paint would know white can lighten a color up. The chapter ends with the catgirls looking up at the stars and saying they need a lot more white if they want change Kuro's black. Another thing we soon learned about the experiments is that they can change forms with the ones we have seen so far being the default cat features along with mice and one-wing. In volume five they mentioned they have other forms they don't remember. Perhaps one of these forms comes with more extreme color manipulation abilities and tapping into it could help aid the black traveler.