There are 3 different types of color corrective services. Changing tone, lightening hair that is too dark and darkening hair that is too light. There are other extreme cases where each hair strand has banding or splotchy results from a previous color service. All color corrective color services require strand testing.  A strand test is when a piece of hair from the client is lightened or darkened to see if the hair can handle another color service. It also helps to determine the porosity and elasticity of the clients hair after color service. This shows if any chemicals previously used on the clients har, have the potential to react negatively with other professional products.