修改UITextfield的Placeholder字体的颜色

    //两种办法
    //attributedPlaceholder
    self.usernameTextField.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"xxx" attributes:@{NSForegroundColorAttributeName: [UIColor redColor]}];
    
    //KVO
    [self.usernameTextField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];