Your hair gets its color from a compound known as melanin, produced by special cells called melanocytes. There are two versions of melanin - one that produces dark pigmentation and the other that produces light pigmentation. If your natural hair color is brown/black, then it normally has a specific amount of dark vs light pigment. The middle section of this hair is grey because there was an absence of melanin production when the hair was still growing. There could be multiple factors (including stress , as described on the library of congress site), but it's nothing to worry about. Your melanocytes started to normally produce melanin again, which is why the newest hair growth (closest to the roots) is dark in color (dark brown/black).