In my experience, I have achieved the best color, and in the least amount of time (less than 4 hours), when mixing the henna with an highly acidic ingredient, i.e. lemon juice, etc. The problem with using such ingredients, however, is that they can be drying on most hair types. For this reason, I would recommend using mildly acidic ingredients like herbal teas (chamomile, green tea, etc) and leave in for at least 6 hours. I hope this helps!