Setting one side of a TableCell border at runtime

I have a Table and am adding TableRows and TableCells dynamically with the bottom border color depending on the data displayed in the cell.
I have not been able to determine how to set just the bottom border color based on the data.

while (reader.Read())
{
TableRow r = new TableRow();

TableCell c1 = new TableCell();
c1.Text = reader["columnName"].ToString();
if(entityID != Convert.ToInt32(reader["ColumnID"].ToString()))
{
c1.BorderColor????? //how do I manipulate just the bottom part of the border
}

etc.....

Thanks for your help.