var d = document.getElementById('id_1491998309944');
d.style.color = '#333333';
Смотрю на вкладку Elements в ChromeDevTools, вижу что у элемента
<tr id="id_1491998309944" draggable="true" style="box-shadow: rgb(76, 175, 80) 0px 0px 3px 7px; color: rgb(51, 51, 51);"><td width="570" class="ree_single_block" style="height: 42px; padding: 10px; color: rgb(51, 51, 51); background-color: rgb(150, 84, 84);"><table width="550" cellpadding="0" cellspacing="0" role="presentation"><tbody><tr><td width="550" class="editable mce-content-body mce-edit-focus" height="42" style="font-weight: bold; text-align: center; position: relative;" id="mce_0" contenteditable="true" spellcheck="false"><h1>Header</h1></td></tr></tbody></table></td></tr>