If your hair is very oily or has a lot of products on it, then you can wash it. Otherwise washing or not washing will not affect how the hair color will turn out. Sometimes shampooing right before hair coloring, especially during hot summer months may make the skin sting slightly if you do a single process. If you have an event you are going to within a day or two of your color and you do a single process do not wash your hair. The less oil you have on your skin, the more you will have a tendency to stain, even if your colorist is very careful about cleaning your hairline. Oil in your skin helps to protect against staining.