swift3_prepare_segue.swift

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if (segue.identifier! == "CustomSegue") {
            //with Navigation Controller
            //let navController = segue.destination as! UINavigationController
            //let detailController = ((navController.topViewController) as! SecondViewController)
            //without NavigationController:
            let detailController = (segue.destination as! SecondViewController)
            detailController.colorstring = "purple"
        }
    }