There are four colours differentiating the Status of an Invoice.
Blue indicates the Invoice is Unissued. An invoice is unissued if it has not been emailed, saved as a PDF or printed.
Yellow indicates that the Invoice has been Issued. All invoices are issued after they have been emailed, saved as a PDF, printed or manually marked as issued.
Green indicates that the invoice has been fully Paid.
Red indicates that the Invoice remains Unpaid or Overdue.