see also Applying Polish
Neutral polish differs from black polish in that it contains no (or at least minimal) pigment. Neutral is used in instances where one does not have the appropriate color polish for a pair of boots. Neutral can also be used in cases where there are multiple colors of leather or in cases where contrastingly colored stitching makes the use of colored polish difficult. Some bootblacks also use netural polish as a top coat on other colored polishes.
Beyond the pigment difference, neutral polish is similar in terms of wax hardness.