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You've probably heard the advice from friends, or even read it in the instructions included in some packages of hair coloring: Don't wash your hair before you color. Weird, huh? After all, dirty hair usually isn't good for much, and the best results in other areas of hair care come with hair that's freshly shampooed. For some reason, though, stylists and home colorists alike swear by unwashed hair. If you're wondering what gives, check out some of the science behind your hair's biology and the mechanics of coloring.
To understand how hair dyes work, understand a little about hair itself. Each hair on your body is made from two parts: an inner, opaque colored core and an outer layer of material that's completely clear and colorless. Your hair's natural color is contained in that core, but when you color your hair, you're usually soaking dye into the outer area. Each hair grows within a pore of your skin, which releases an oily substance called sebum (this is the same stuff that gives you a greasy complexion and cause pimples). This oil will get into your hair and coat it, especially where the hair touches the head.
Hair Oils Help
So why does unwashed hair get better dye results than freshly washed? Well, it's not actually dirty hair you want, but oily -- specifically from the natural oils excreted by your skin. This oil will catch the dyes and hold them more thickly in place against the individual hairs, much in the way that thick cough syrups coat your throat. When you shampoo hair, you apply detergents that break up oil and wash it away, so the more recently your hair was washed, the less oil there is. But if your hair is full of dirt, dust or hair products, these things will actually get in the way of the dye.
Optimal Dye Conditions
Basically, what all this means is that the best conditions for dyeing your hair are when your hair is unwashed, but not too terribly dirty. You'll have a good buildup of those natural oils, but not too much dirt and grime that could end up getting in the way of the dye. The natural oiliness of your scalp and whether you have dry or oily skin will make a difference in the results you get.
Prepping for Dye
Try to see if you can go for somewhere between 24 and 48 hours without washing, but only if you can avoid dusty, dirty conditions, and keep from putting product in that hair (yes, it's a tall order). A good way to do this is to wash your hair in the morning, wear it up for a day (no need for products), then go the second day in a bandana or hat before dyeing at night. If you need to spritz your hair with a little cool water to help control it, no problem; a little sprayed water alone won't disturb the buildup of those oils.