Excel.Application application = new Excel.Application();
Excel.Workbook workbook = application.Workbooks.Open(@"C:\Test\Whatever.xlsx");
Excel.Worksheet worksheet = workbook.ActiveSheet;

Excel.Range usedRange = worksheet.UsedRange;

Excel.Range rows = usedRange.Rows;

int count = 0;

foreach (Excel.Range row in rows)
{
    if (count > 0)
    {
        Excel.Range firstCell = row.Cells[1];

        string firstCellValue = firstCell.Value as String;

        if (!string.IsNullOrEmpty(firstCellValue))
        {
            row.Interior.Color = System.Drawing.Color.Red;
        }
    }

    count++;
}

workbook.Save();
workbook.Close();

application.Quit();

Marshal.ReleaseComObject(application);