Here are the beginning stages of a digital "painting" of me with my mother. I use the same approach in all these paintings. It's really the same approach as if I were working in oil paint.
In digital art you can make use of layers. In this case there is an initial sketch layer that can be turned on and off. Tones and local colors are added in a level below the sketch layer.
"Mistakes" can be deleted by back spacing steps. The stylus also has an eraser which is helpful in the sketch stage.
If there is a reference photo or color sketch, a "color picker" in the software program allows you to touch the color area of the photo to copy that color so you can paint with that color/tone. Fulfills the same function as color/tone matching the traditional way by mixing and holding up the brush loaded with the color to compare with what you see in the original scene/subject.