Currently I am working to learn how to use Gtk3 with Python 3.6. So far I have been able to use a combination of resources to piece together a project I am working on, some old 2.0 references, some 3.0 shallow reference guides, and using the python3 interpreters help function.
However I am stuck at how I could customise the statusbar to display a progressbar. Would I have to modify the contents of the statusbar to add it to the end(so it shows up at the right side), or is it better to build my own statusbar?
Also how could I modify the progressbars color? Nothing in the materials list a method/property for it.
GtkStatusbar is a subclass of GtkBox. You can use any GtkBox method including pack_start and pack_end or even add, which is a method of GtkContainer. 
Thus you can simply add you progressbar to statusbar.
Python Gtk3 - Custom Statusbar w/ Progressbar