With temporary dyes I must leave on the roots 5-10 minutes longer. I have dark brown hair and usually go a shade darker also. With henna and indigo you must clairify , especially the roots to get the oils off and make sure you do the 2-step process and use Yemen henna and good indigo. You could always try this and if it doesn't work you could do the temporary dye job!
Type: 3a, fine, thin, low porosity Water/Climate: soft/ marine Washes: water w/washcloth, shikakai, clay Condition: Style:Seamollient, CurlsPFCCP, Miracurl, Gel-les'c, CK; rudeechick's super soaker method; blot, air dry 30 min; diffuse Also: coconut oil