Untitled

<table>

  <tbody>

    <tr>

      <td>a</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>b</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>c</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>a</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>b</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>c</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>a</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>b</td>

      <td></td>

      <td></td>

    </tr>

    <tr>

      <td>c</td>

      <td></td>

      <td></td>

    </tr>

  </tbody>

</table>

td {

  border: 1px solid;

  min-width: 5em;

}

tr:nth-child(8n+1),

tr:nth-child(8n+2),

tr:nth-child(8n+3),

tr:nth-child(8n+4) {

  background: orange;

}