Hi,
recently I had a same problem, so I try to help.
The style of 'error' should be still defined in the template.css so there is no need for declarate it again.
You could look for it in the template editor in the template.css file. If there is not such a definition you can create one there: e.g.
Code:
.error_customer{color:red;}
.
Then you can adress this directly with the solution of DenisChenu:
Code:
<td class="{if(Q1!='A4','error_customer','')}">Q1</td>
But I think it should work without an own class.
Best regards
Stefan