Single Processing = A color process done in one application. (Book this service for Jet Black or if you have 613 hair that you want turned to one specific color like lime green for instance ) Double Processing = A color process done in two applications. (Book this service when you have dark hair and want it turned lighter with an additional color.) For instance.. To achieve fire red with dark hair we'd have to lighten the hair first with bleach then color it red. Multiple Processing = A color process done in multiple applications. (Book this service for multiple colors on one unit). For instance you may want a 613 unit colored with more than two colors or if you want highlights. Turn around time: 7-14 business days possibly sooner depending on order volume & color job requested.