Обновлен Swift 4.1
UISegmentedControl.appearance().setTitleTextAttributes([NSAttributedStringKey.foregroundColor: UIColor.red], for: .selected)
Swift 3.1
UISegmentedControl.appearance().setTitleTextAttributes([NSForegroundColorAttributeName: UIColor.red as Any], for: .selected)
Для более ранней версии Swift:
UISegmentedControl.appearance().setTitleTextAttributes([NSForegroundColorAttributeName: UIColor.red], for: .selected)