You may want to try eclipta alba. It is an ayurvedic herb that darkens hair. You can buy it in a hair oil. I have never used it myself, I just came across it while researching herbs. If you do try it, please post results. I may put some in my husband's shampoo.
CurlyFireHorse
Hair: 2a or 3a (not sure), very porous and fine as frog hair
Day 1: Cowash and condition with AO GPB, ACV Rinse,
Co-Kra gel (this stuff is fantastic)

Day 2: KCKT, KCCC, plop, KCCC, plop again, air dry
Day 3: Curly Kinks Satin Roots, Coil Jam
Day 4: IAGirl's PT, KBB DT, FSG

Co-Kra gel is my own concoction of okra boiled in coconut milk. It is a cross between FSG and KCCC and leaves my hair very soft and shiny