I'm trying to set different back color of my DayaGridview rows by specific column

This is my Code:

private void SetFormat_dgv_Scheduled(string columnsname)
    {
        dgv_Scheduled.Sort(dgv_Scheduled.Columns[columnsname], ListSortDirection.Ascending);
        Color clr = Color.Red;
        string dt = (Convert.ToDateTime(dgv_Scheduled.Rows[0].Cells[columnsname].Value)).ToShortDateString();
        foreach(DataGridViewRow row in dgv_Scheduled.Rows)
        {
            if (Convert.ToDateTime(row.Cells[columnsname].Value).ToShortDateString() != dt)
            {
                if(clr==Color.Red)
                {
                    clr = Color.LightBlue;
                }
                else
                {
                    clr = Color.Red;
                }
            }     
             row.DefaultCellStyle.BackColor = clr;
        }          
    }

It works fine.

But when I click header to autosort, all format will set to default style.

(All red rows change back to white)

Is there any way that I can set format unchangeable?

Thanks.