iOS 7 UIBarButton back button arrow color

You have to set the tintColor of the entire app.

self.window.tintColor = [UIColor redColor];

Or in Swift 3:

self.window?.tintColor = UIColor.blue

Source: iOS 7 UI Transition Guide

To change the back button chevron color for a specific navigation controller*:

self.navigationController.navigationBar.tintColor = [UIColor whiteColor];

*If you are using an app with more than 1 navigation controller, and you want this chevron color to apply to each, you may want to use the appearance proxy to set the back button chevron for every navigation controller, as follows:

[[UINavigationBar appearance] setTintColor:[UIColor whiteColor]];

And for good measure, in swift (thanks to Jay Mayu in the comments):

UINavigationBar.appearance().tintColor = UIColor.whiteColor()