Backgrounds for UInavigationBar and UIPickerView

My understanding is that when you create a navigation controller it has a navigation bar, you can only select three backgrounds for that (example: semi-transparent black, etc.). I've seen applications that have other colors, such as green with a semi-transparent black feature. Are they just creating their own background view and setting it to the UINavigationBar? how do they get this "translucent" effect? like the whole view, it is not one color, but the top looks like a transparent transparent, and the bottom looks like another. Similar to UIPickerView, which has a nice design in the background.

How can i do this? to design graphics with graphic design? or just use quartz (gradients, etc.) on the views to get this?

Thank!