方式一：
_codeTextField.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"请输入验证码(4位数字)" attributes:@{ NSForegroundColorAttributeName:[UIColor colorWithHex:kTextLightGray]}];
方式二：KVC方式
_phoneTextField.placeholder = @"请输入手机号码";
[_phoneTextField setValue:[UIColor colorWithHex:kTextLightGray] forKeyPath:@"_placeholderLabel.textColor"];