Datagridview Cell Back color returs null

Hello, the issue is the way you are using the cell.Style property.

DataGridViewCell on a row that do not have their own styling use the InheritedStyle property of the DataGridViewRow they are on.

Try this minor change:

foreach (DataGridViewRow row in DGV_Result.Rows)
{
    int colindx = 0;
    rowindx++;
 
    foreach (DataGridViewCell cell in row.Cells)
    {
        colindx++;
        ws.Cells[rowindx, colindx].Value = cell.EditedFormattedValue;
        MessageBox.Show(row.DefaultCellStyle.BackColor.Name);
        if (cell.InheritedStyle.BackColor == Color.Red)
        {
            ws.Cells[rowindx, colindx].Style.Fill.BackgroundColor.SetColor(Color.Red);
        }
        else if (cell.InheritedStyle.BackColor == Color.Yellow)
        {
            ws.Cells[rowindx, colindx].Style.Fill.BackgroundColor.SetColor(Color.Yellow);
        }
        else if (cell.InheritedStyle.BackColor == Color.Green)
        {
            ws.Cells[rowindx, colindx].Style.Fill.BackgroundColor.SetColor(Color.Green);
        }
        else
        {
            ws.Cells[rowindx, colindx].Style.Fill.BackgroundColor.SetColor(Color.White);
        }
    }
}

Hope that helps.