To dye hair and get the best results hair should not have been washed in the past 24 hours. However, it is also important that it is absolutely free of any styling products as well.
Many shampoos and conditioners can coat the hair or cause the color to come out completely different than what is desired. Wash hair 24 hours before coloring, and do not put any products in the hair. Styling products can also coat strands of hair, making it impossible for hair dye to penetrate hair strands, which also leads to undesirable colors or hair color that fades quickly.