I'm having trouble adding the lightbox button to my website.

I am sorry for the trouble caused to you. There seems to be some confusion as the color you are changing is for the lightbox title background & text color. Please check the screenshot below:

If you want the background color of the hyperlink to be purple and font color to be white, we need to edit the embed code. Please use the following lightbox code and that should solve your problem:

<script src="https://form.jotform.com/static/feedback2.js" type="text/javascript"></script><script type="text/javascript">

        var JFL_50623256188962 = new JotformFeedback({

          formId: '50623256188962',

          base: 'https://form.jotform.com/',

          windowTitle: 'Register NOW',

          background: '#9013fe',

          fontColor: '#ffffff',

          type: 'false',

          height: 500,

          width: 700,

          openOnLoad: false

        });

    </script>

      <a class="btn lightbox-50623256188962" style="margin-top: 16px;background-color: #9013fe;color: #fff;">

        Course A Evaluation- Lessons 11-20, American People II

      </a>

Hope this helps.

Do get back to us if you have any questions.

Try adding this code "padding-top: 7px; padding-bottom: 7px;" in the <a> tag:

<script src="https://form.jotform.com/static/feedback2.js" type="text/javascript"></script><script type="text/javascript">

        var JFL_50623256188962 = new JotformFeedback({

          formId: '50623256188962',

          base: 'https://form.jotform.com/',

          windowTitle: 'Register NOW',

          background: '#9013fe',

          fontColor: '#ffffff',

          type: 'false',

          height: 500,

          width: 700,

          openOnLoad: false

        });

    </script>

      <a class="btn lightbox-50623256188962" style="margin-top: 16px;background-color: #9013fe;color: #fff;padding-top: 7px; padding-bottom: 7px;">

        Course A Evaluation- Lessons 11-20, American People II

      </a>

I am sorry, but we have not received your screenshot. We can not receive attachments sent with email reply. Could you please visit this support thread to upload a screenshot? 

Guide: How-to-Post-Screenshots-to-Our-Support-Forum

It would help us a lot if you can share web site link so we checked embedded form.

Thank you!

Update the button code with the following.

<a class="btn lightbox-90736080327961" style="margin-top: 16px;background-color: #cf1e25;color: #fff;padding: 10px 20px;text-decoration: none;display: inline-block;cursor:pointer;">Reserve Your Seat!</a>

Result: