By far the hardest thing for most stylists is colour correction. There are so many other factors in hair colour than just picking a colour out of a swatch book, throwing it on someone’s head, and getting the results that you wanted.
Here is a classmate of mine. She had put black colour over her previous platimum blonde heavy weave. Now she is wanting to go back to her natural hair colour. The process took several hours and included tint remover, Redken’s Colour Fusion, and Shades EQ for toning.

Here is the before shot. You can see about 1.5" of natural regrowth and her black colour.

Here is the after shot. A beautiful chocolate brown.

An Outside Shot
Now, she is wanting to work in a few caramel / honey highlights so keep stopping by to see the updates.