I wanted a little escape tonight so I went in the App store and purchased the Dr. Who colouring book. It took about 30 seconds to realize that purchase was a mistake. It isn't a coloring book at all. Instead, you get a picture and a series of colors. You then tap in a white space and it fills in with the selected color … perfectly … every time. Without the actual act of drawing, I don't get the expected therapy.
If you're looking for a coloring book app (and if you have an iPad Pro and an Apple Pencil I recommend you at least try), give Pigment a try. It gets iPad coloring right.