It is a common misconception that it is better to come in with "dirty" hair before a color service.  In fact, coloring clean hair results in a more even, longer lasting color, and better grey coverage.  The only benefit of not having clean hair before a color service would be that your own natural scalp oils can work as a bit of a protective barrier between your skin and the products, and this is only really recommended if you're receiving an all-over lightening service or have an especially sensitive scalp.