Changing the bar colour of a Gtk progress bar

[John Emmas <johne53 tiscali co uk>]

Suppose I have a Gtk progress bar with some text displayed on it.  How do I change the colour of the moving 
bar?  So far I've tried gtk_widget_modify_bg (which sets the background colour, as you'd expect), 
gtk_widget_modify_fg (which sets the text colour) and gtk_widget_modify_base (which appears to do nothing).

What's the secret for changing the colour of the moving progress bar??

John