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I have found clean to be the best. Im not a pro but i have done my hair a million times and this is my thinking.
If i wash dry and then dye my hair is not at healthy feeling.
If i dont wash for 2 days the color is not as nice.
If i wash during the day and dont do much to get me nasty and then dye later that night or wash b4 bed and dye in the morning I get nice hair.
I think you want some natural oils on your hair to protect it but if its to dirty it will have a hard time coloring your hair because you will have oil and dirt blocking the pores of your hair (if they are called pores).
Like i said Im not a pro but from doing my own hair and friends hair so many times thats what i have come up with.
The worst job i did was quite a few years ago, i had been outside all day jerking around and came in after dark to do my hair, I used 2 bottles covered every bit of my hair. It was bad, i had placed that didnt hold any color, places that were not even close to the same color. It was a mess for a while.
The best job i did was a black and purple job. I had washed and gone to bed, It was a cool fall night so i didnt sweat. When i woke up I went to work and it was perfect. The best black and purple I had done. I had been doing it for a year before that day and kept it almost 2 years after.