While there is some controversy about this question, it is a fact that it takes longer for the chemicals to break through dirt to perform the reaction desired. If the hair is not too dirty, the dirt will help protect the scalp from the sometimes harsh reaction to the skin. "Too dirty" is a relative term, and includes styling products and processes.
If your hair has been washed within 3 days and you have not used a whole can of hairspray on it, then you do not need to re-wash your hair prior to coloring it. It will not affect the chemical process.
They say when you're hair is dirty the hair color comes out
better. But maybe they mean by when you've put some products in
your hair or something, and haven't washed it in like a day or
When its dirty! When you apply dye to the newly washed hair shaft it WILL take BUT by washing it you have stripped it of all its natural oils WHICH is better to have present on the hair in order for the dye to adhere easier. If you've ever had your hair dyed at a salon you'll have noticed that they NEVER EVER wash the hair first, they just start applying it no matter WHAT dirt or products have built up on it. So nevermind what the over-the-counter drug store hair dye instructions recommend, DIRTY is better!!!