Design Detail: Buttons that communicate intention

I came across this example of an iPhone application that uses some great, simple design with its interface. Run Keeper is an app that tracks your runs (or bikes, or hikes, or walks, or…). When you first start out you get this really nice big green button. When you are done you get and equally nice red button. They could have gone with a more standard blue shade on the button, but it wouldn’t say anything about what that particular piece of interface did. A nice, thoughtful touch.