Wait at least 24 hours to shampoo, or preferably longer. During this period upon color-treating your hair, the hair color gradually settles into the hair strands. By avoiding shampooing, you give the color plenty of time to set which makes it less likely for it to fade. As part of most hair coloring services, your hairstylist would have washed your hair before treating it. Nonetheless, if you still desire to give your hair another wash on the day of the hair color, you should only run cold water through the strands.