New on her blog Elizabeth Smith talks about tree views and rendering in a PHP-GTK application (with sample code included).
I get this question a lot - "How do I change the background color of a single cell in a treeview?" If you look around you'll see a couple of ways of doing this using display callbacks and other highly processor and memory intensive methods - but there's a much simpler way.
She explains how the tree views are rendered (with the GtkTreeView component, of course) via a "cell renderer" with different variations based on the contents of the cell - each with their own properties. Her example code illustrates how to manipulate this by making the tree object and attaching other columns to it with custom background properties. A quick screencast is included to show it in action.