Try any one of this for change the back color of header row
GridView1.HeaderRow.Cells[0].BackColor = Color.Red;
GridView1.Columns[0].HeaderStyle.BackColor = Color.Red;
Try this for change the text color of header row
GridView1.HeaderRow.Cells[0].ForeColor = Color.Red;
GridView1.Columns[0].HeaderStyle.ForeColor = Color.Red;