The idea is to fill an area by coloring the fabric behind the embroidery. Usually, this means that only the outline is embroidered, with some details added. However, tinting isn't only for coloring inside the lines. Many artists use this technique to create spectacular backgrounds, borders, and accents for their embroidery work.
This art therapy trend has yet to hit America, but because publishers have found it immensely profitable, it's probably only a matter of time before you find yourself shopping for a new box of crayons. And hey, why not? Coloring is a fun and joyful activity that is a much easier way to reduce stress than, say, dragging yourself to the gym. It just might get awkward if your friends find your completely finished Benedict Cumberbatch coloring book.