A one color process refers to any color service that is done in one step. It can also be referred to as a base color or a "root touch up", but it is not restricted to coloring just on the "roots". This is generally a permanent color that is applied either jus to the roots or all over from roots to ends. a single process can also be a demi-permanent color or a glaze, which is a type of color that only deposits color into the hair.
