Always Color in a Consistent Direction. This is my first tip. It will make a bigger impact on the end result than anything else and any age can use it! The changes in direction, shown on the red kidney below, on the left side, are what make it look sloppy, even though I carefully stayed in the lines. Coloring in a consistent direction, as seen on the other kidney, is easier on the eyes and ends up looking very tidy!
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.