You can use css (GTK3) to change the colors, something like:
style_provider = Gtk.CssProvider()
css = '''
GtkTreeView row:nth-child(even) { background-color: shade(@base_color, 0.9); }
GtkTreeView row:nth-child(odd) { background-color: shade(@base_color, 1.0); }
'''
style_provider.load_from_data(css.encode('utf8'))
Gtk.StyleContext.add_provider_for_screen(Gdk.Screen.get_default(),
style_provider,
Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION)
Make sure to tell GTK that you want to paint alternate colors:
treeview.set_rules_hint(True)