This is a new direction since buying an iPad (after drooling and deliberation). The app used is Brushes. Its such a great way to lay colour down quickly and the emphasis is on colour as its so brilliant at all the subtle choices and changes in tone and hue. I've chosen not to try and be super realistic, but rather work with the colour side of it and try to be simple in style, working instinctively.
Here's the Brushes site: