Inkflow is available free for the iPhone and iPad.
The premium version – Inkflow Plus – has a few more tools.
Inkflow is advertised as a way to make visual notes quickly, but I’ve been using it as a way to paint with my finger on my iPhone.
Here’s a painting I made. I did it from scratch, copying the figure of Madam Butterfly in a poster we have on the wall.
The working area on an iPhone is pretty small of course, but you can pinch the screen open to work within a smaller area of the painting.