puedes hacerlo asi:
Código PHP:
$r=1;
do{
if($r%2==0)
{
echo "<table cellpadding='3' cellspacing='0' border='0' width='100%'>";
echo "<tr bgcolor='#CCCC66'>";
echo "<td width='50' height='20'><img src='/mf_temp/images/unread_icon.gif' alt='' /></td>";
echo "<td width='408'>Ultimos diez usuarios: ".$row_sql['nick']." </td>";
echo "</tr>";
echo "</table>";
}
else
{
echo "<table cellpadding='3' cellspacing='0' border='0' width='100%'>";
echo "<tr bgcolor='#FFFFFF'>";
echo "<td width='50' height='20'><img src='icon.gif' alt='' /></td>";
echo "<td width='408'>Ultimos diez usuarios: ".$row_sql['nick']." </td>";
echo "</tr>";
echo "</table>";
}
++$r;
}while($row_sql = mysql_fetch_assoc($mostrar_sql));
Saludos.