It’s quite a challenge to draw with my fingers instead of a stylus or other tool, but Sketchbook Pro makes it pretty easy with lots of ways to quickly zoom and move the canvas around. I tried two markers, one fine and one thick, set to about 80% opacity and another softer brush for the colouring.
Had a bit of a scare when opening the file when we got home and found the layer with the lines was missing! Closing (without saving) and re-opening through the Gallery recovered the file properly, but it was disheartening for a few minutes. The application exports as a PSD with layers, which is an excellent option.
Looking forward to trying out more of the brushes and blending some colours. I would still prefer a stylus, though, with pressure sensitivity.