private void dataGridView1_CellMouseDoubleClick(object sender, DataGridViewCellMouseEventArgs e)
        {
            dataGridView1[e.ColumnIndex, e.RowIndex].Tag = "zzzz";
        }

        private void dgv_main_CellPainting(object sender, DataGridViewCellPaintingEventArgs e)
        {
            if (e.ColumnIndex >= 0 && e.RowIndex >= 0 && 
                dataGridView1[e.ColumnIndex, e.RowIndex].Tag != null &&
                dataGridView1[e.ColumnIndex, e.RowIndex].Tag.ToString() == "zzzz")
            {
                e.PaintBackground(e.ClipBounds, true);

                using (Brush gridBrush = new SolidBrush(System.Drawing.Color.Blue), backColorBrush = new SolidBrush(dataGridView1[e.ColumnIndex, e.RowIndex].Style.BackColor))
                {
                    using (Pen gridLinePen = new Pen(gridBrush, 2))
                    {
                        e.Graphics.DrawLine(gridLinePen,
                                            e.CellBounds.Left + ((e.CellBounds.Right - e.CellBounds.Left) / 2), e.CellBounds.Top,
                                            e.CellBounds.Left + ((e.CellBounds.Right - e.CellBounds.Left) / 2), e.CellBounds.Top + ((e.CellBounds.Bottom - e.CellBounds.Top) / 3));
                    }
                }
                
                e.Handled = true;
            }
        }