<div class="row mt-5">
    <div class="col">
    </div>
    <div class="col mt-5">
        <%= form_tag create_path, method: :post do %>
            <div class="form-group">
              <%= label_tag :team_name, 'Team Name' %>
              <%= text_field_tag :team_name, '', class: 'form-control', placeholder: 'GOV.UK Signify' %>
            </div>
            <div class="form-group">
              <%= label_tag :colour, 'Background colour (optional)' %>
              <a href="http://leaverou.github.io/contrast-ratio/#white-on-purple">How to check your colour choice is readable</a>
              <%= text_field_tag :colour, '', class: 'form-control', placeholder: "purple" %>
            </div>
            <div class="text-center mt-5">
                <%= submit_tag 'Signify!', class: 'btn btn-primary btn-lg' %> 
            </div>
        <% end %>
    </div>
    <div class="col">
    </div>
</div>

<div class="row mt-5">
    <div class="col">
    </div>
    <div class="col mt-5">
        <%= form_tag create_path, method: :post do %>
            <div class="form-group">
              <%= label_tag :team_name, 'Team Name' %>
              <%= text_field_tag :team_name, '', class: 'form-control', placeholder: 'GOV.UK Signify' %>
            </div>
            <div class="form-group">
              <%= label_tag :colour, 'Background colour (optional)' %>
              <%= text_field_tag :colour, '', class: 'form-control', placeholder: "purple" %>
            </div>
            <p>
              <a href="http://leaverou.github.io/contrast-ratio/#white-on-purple">
                How to check your colour choice is readable
              </a>
            </p>
            <div class="text-center mt-5">
                <%= submit_tag 'Signify!', class: 'btn btn-primary btn-lg' %> 
            </div>
        <% end %>
    </div>
    <div class="col">
    </div>
</div>

Move help text below form
Move help text below form
