I don't the direct way of doing this but you can achieve this by adding a UIBUtton to the customview of the UIBarButtonItem.
UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
backButton.frame = CGRectMake(0, 0, 80, 40);
[backButton setBackgroundImage:[UIImage imageNamed:@"back-button.png"] forState:UIControlStateNormal];// Setting the background Image
[backButton setTitle:@"Back" forState:UIControlStateNormal];//Setting the title of the button
[backButton setTitleColor:[UIColor redColor] forState:UIControlStateNormal];// Setting the text color.
[backButton addTarget:self action:@selector(backButtonTaped) forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *backButtonItem = [[UIBarButtonItem alloc]initWithCustomView:backButton];
self.navigationItem.leftBarButtonItem = backButtonItem;
And this will give you a UIBarButtonItem with the text "Back" in red color.