Not familiar with RubyMotion, but you should use the UIAppearance proxy:
[[UINavigationBar appearance] setBarTintColor:[self navBarColor]];
[[UINavigationBar appearance] setTintColor:[self navBarTitleColor]];
[[UINavigationBar appearance] setTitleTextAttributes:@{NSForegroundColorAttributeName : [self navBarTitleColor], NSFontAttributeName : [self titleFont]}];
You should do this before you load your main window in your AppDelegate.