I think this app (although nice looking) misses the point of coloring book exercises. I think that using a pen/cil to color is not only aimed at fostering kids' creative process but also developing their hand-eye coordination and improve on their focus. I dont really have the time to dig out the references for this right now.

An additional side effect of this app is that now coloring takes less time. A great feature of coloring books is their ability to "buy" you (the parent) 30-60 minutes of peace.