This particular app makes use of the smartphone's touchscreen, and this image has been drawn with the artist's relatively thick fingertip (compared to the smartphone screen, that is).
The resulting lack of precision in the mark-making can provide an enjoyable challenge, and the end product can be quite aesthetically appealing as a result of the continual process of refinement. Colin has translated his traditional sculptural charcoal-drawing style over into the digital medium to create this image."
Ed -The Model was Adrian - with the addition of another 20 years, the likeness is Very good. Must have a go!