My sister has been natural about 3 years and her hair is BSL. She made the big mistake of buying over the counter black permanent hair color and colored her hair. The color did not cause any damage, but now she's sick of it and whats to try henna. Can you apply or should you apply henna over black permanent color. If you do, what type of results can you expect. Will she still get the conditioning benefit? Will the color of her hair be effected at all? Would hate to see her with green hair.

Any advice is appreciated