如圖：由于label1和label2在同一位置，這是在上層的label2顏色顯示就會有偏差，正常應該是label3顯示效果

    UILabel *label1 = [[UILabel alloc] initWithFrame:CGRectMake(50, 100, 100, 20)];
    label1.text = @"兌獎券";
    label1.textColor = [UIColor redColor];
    [self.view addSubview: label1];
    
    UILabel *label2 = [[UILabel alloc] initWithFrame:CGRectMake(50, 100, 100, 20)];
    label2.text = @"兌獎券";
    label2.textColor = [UIColor whiteColor];
    [self.view addSubview: label2];
    
    UILabel *label3 = [[UILabel alloc] initWithFrame:CGRectMake(150, 100, 100, 20)];
    label3.text = @"兌獎券";
    label3.textColor = [UIColor whiteColor];
    [self.view addSubview: label3];