This is a very rough, but common approach to this problem. If the person draws back and forth, more will be laid down and you will get a darker (more solid/less transparent) color. Then as the user drags their finger across the device record the path and draw these "splotch" images along the path. Photoshop, Fireworks, and many other drawing applications already have brushes and line styles for crayon like effects, so you can simply play around with a few of those on a transparent background to get some workable "splotches" (small round crayon looking transparent PNGs). The easiest implementation to get your head around, at least just getting started, would be to create a series of transparent crayon textures. The standard Quartz calls that you would use in drawRect: won't actually give you the crayon effect that your looking for using any default behavior.
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