I've put 2-4 tbsp of coconut oil in my henna mix before with no issues. Some people report less dye uptake that way, but I didn't have issues. Also, I've heard that using coconut oil prior to coloring, however you color, can protect it but I've never done that myself.
My usual mix is my Rainbow henna dark brown, with a few squirts of camellia oil and 2 tbsp organic ACV, all mixed with a hibiscus based tea. It works well.
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