Swift：设置背景颜色和标签文本颜色

Question

我正在尝试更改标签文字颜色并查看背景颜色。但是出了点问题。

    func ItisDark(){
        self.view.backgroundColor = UIColor.blackColor()
        self.navigationController?.navigationBar.barStyle = UIBarStyle.Black
        self.navigationController?.navigationBar.barTintColor = UIColor.blackColor()
        self.navigationController?.navigationBar.tintColor = UIColor.whiteColor()
        questionLabel.textColor = UIColor.whiteColor()
        answerLabel.textColor = UIColor.whiteColor()
        infoBtn.tintColor = UIColor.whiteColor()
    }

func checkDate(sunrise:NSDate, sunset:NSDate){
    // Date comparision to compare current date and end date.
    let dateComparisionResult:NSComparisonResult = NSDate().compare(sunrise)
    let dateComparision:NSComparisonResult = NSDate().compare(sunset)

    if dateComparisionResult == NSComparisonResult.OrderedSame || dateComparisionResult == NSComparisonResult.OrderedDescending && dateComparision == NSComparisonResult.OrderedAscending
    {
        //result is
        answerLabel.text = "yes".uppercaseString
        self.view.backgroundColor = UIColor.whiteColor()
        questionLabel.textColor = UIColor.blackColor()
        answerLabel.textColor = UIColor.blackColor()
    }else{
        answerLabel.text = "no".uppercaseString
       //here is the problem!
        ItisDark()

    }

}

我在空项目中尝试了这段代码。一切都很完美。但是我在另一个项目中看着白屏。你在想什么问题？