gistfile1.php

<?php

// At top of template_preprocess_table function for test
 $colgroups = array(
      // COLGROUP with one COL element.
      array(
        'data' => array(
          array(
            'class' => array('test1-1'), // Attribute for the COL element.
          ),
           array(
            'class' => array('test1-2'), // Attribute for the COL element.
          ),
        ),
        'class' => array('jazzy1'), // Attribute for the COLGROUP element.
      ),
      // Colgroup with attributes and inner COL elements.
      array(
        'data' => array(
          array(
            'class' => array('test2-1'), // Attribute for the COL element.
          ),
           array(
            'class' => array('test2-2'), // Attribute for the COL element.
          ),
        ),
        'class' => array('jazzy2'), // Attribute for the COLGROUP element.
      ),
    );


 ?> 

// IN table.html.twig
{% if colgroups %}
    {% for colgroup in colgroups %}
      {% if colgroup.cols %}
        <colgroup{{ colgroup.attributes }}>
        {% for i,col in colgroup.cols %} 
         <!-- {{ dump(col.attributes.class ) }} -->
         <col{{ col.attributes }}/>
        {% endfor %}
        </colgroup>
      {% else %}
        <colgroup{{ colgroup.attributes }}/>
      {% endif %}
    {% endfor %}
  {% endif %}



  <colgroup class="jazzy1">
         
         <!-- <pre class='xdebug-var-dump' dir='ltr'>
<b>object</b>(<i>Drupal\Core\Template\AttributeArray</i>)[<i>505</i>]
  <i>protected</i> 'printed' <font color='#888a85'>=&gt;</font> <small>boolean</small> <font color='#75507b'>false</font>
  <i>protected</i> 'value' <font color='#888a85'>=&gt;</font> 
    <b>array</b> <i>(size=1)</i>
      0 <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'test1-1'</font> <i>(length=7)</i>
  <i>protected</i> 'name' <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'class'</font> <i>(length=5)</i>
</pre> -->
         <col class="test1-1"/>
         
         <!-- <pre class='xdebug-var-dump' dir='ltr'>
<b>object</b>(<i>Drupal\Core\Template\AttributeArray</i>)[<i>836</i>]
  <i>protected</i> 'printed' <font color='#888a85'>=&gt;</font> <small>boolean</small> <font color='#75507b'>false</font>
  <i>protected</i> 'value' <font color='#888a85'>=&gt;</font> 
    <b>array</b> <i>(size=1)</i>
      0 <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'test1-2'</font> <i>(length=7)</i>
  <i>protected</i> 'name' <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'class'</font> <i>(length=5)</i>
</pre> -->
         <col class="test1-2"/>
                </colgroup>
                        <colgroup class="jazzy2">
         
         <!-- <pre class='xdebug-var-dump' dir='ltr'>
<b>object</b>(<i>Drupal\Core\Template\AttributeArray</i>)[<i>282</i>]
  <i>protected</i> 'printed' <font color='#888a85'>=&gt;</font> <small>boolean</small> <font color='#75507b'>false</font>
  <i>protected</i> 'value' <font color='#888a85'>=&gt;</font> 
    <b>array</b> <i>(size=1)</i>
      0 <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'test2-1'</font> <i>(length=7)</i>
  <i>protected</i> 'name' <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'class'</font> <i>(length=5)</i>
</pre> -->
         <col/>
         
         <!-- <pre class='xdebug-var-dump' dir='ltr'>
<b>object</b>(<i>Drupal\Core\Template\AttributeArray</i>)[<i>758</i>]
  <i>protected</i> 'printed' <font color='#888a85'>=&gt;</font> <small>boolean</small> <font color='#75507b'>false</font>
  <i>protected</i> 'value' <font color='#888a85'>=&gt;</font> 
    <b>array</b> <i>(size=1)</i>
      0 <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'test2-2'</font> <i>(length=7)</i>
  <i>protected</i> 'name' <font color='#888a85'>=&gt;</font> <small>string</small> <font color='#cc0000'>'class'</font> <i>(length=5)</i>
</pre> -->
         <col/>
                </colgroup>