如何通过xml向android中的按钮添加渐变？

I cannot figure out why this code will not work. Can anyone help? I am trying to get the button to use a custom color titled 'greenstart' and 'greenend'. The colors have been created in the res/value/string.xml file. I have looked at similar questions but either they were left unanswered, unclear, or the solution did not work. Thanks in advance.I cannot figure out why this code will not work