Hi BJ! I had to ask the same question a few years ago… I didn’t know the difference either! Here’s Rita Hazan’s response…
A single process is either permanent or semi-permanent color that alters your natural hair color either darker or lighter. This is just one-dimensional color, meaning your whole head of hair will be the same. For example, I only do a single process on Debra Messing. There are no highlights, so it is very one-dimensional.
A double process is a single process plus highlights. So you color your hair one shade and then add highlights to give it a multi-dimensional look, like Jessica Simpson. For most people this helps it look more natural. She colors her natural hair to a golden blonde and highlights on top of it with baby blonde highlights for a more natural blonde affect.
Have questions for Rita Hazan? Send them to me at firstname.lastname@example.org!
**Photos Courtesy of Glam**