A single process refers to any color service that is done in one step. This is just one-dimensional color, meaning your whole head of hair will be the same. This can be using a permanent color that lifts and deposits, a glaze, highlights/lowlights without toning, or a creative color service with only one process.
Prices start at $90
A double process color refers to anytime two color services are done in one visit. Generally this is done by doing the first color service, washing and drying the hair, then doing the second color. This can include lightening the hair then applying a toner, or doing a permanent color followed by a glaze.