Displaytag css colocar linha vermelha apenas na header da table

Você pode configurar o css.

Style Sheets

The display:table tag produces well formed html tables with and

sections. Css classes are also automatically added to rows/cells when needed.

You can easily customize the generated table simply specifing appropriate css rules in your stylesheet, based on these classes/selectors.

All the automatically added css classes can be customized in displaytag.properties. You can also add a css class to any generated display:table if needed. This is the list of css classes added by default:
class assigned to
odd assigned to the tr tag of all odd numbered data rows
even assigned to the tr tag of all even numbered data rows
sorted assigned to the th tag of the sorted column
order1 assigned to the th tag of the sorted column if sort order is ascending
order2 assigned to the th tag of the sorted column if sort order is descending
sortable assigned to the th tag of a sortable column

http://displaytag.sourceforge.net/11/tut_style.html