Custom colors for tableView Cells

Cocoa Programming for OS X (5th Edition)
1

I have the following code to display macOS tableview…

func tableView(_ tableView: NSTableView, viewFor tableColumn: NSTableColumn?, row: Int) → NSView? {

// let currentItem = invoiceItems[row]

        let currentItem = customerinvoices[row]

/// print(customerinvoices[row].invoiceNumber!)
var text: String = “”
var cellIdentifier: String = “”
tableView.backgroundColor = .gray

        let currentColumn = tableColumn?.identifier.rawValue
        if currentColumn == fieldIdentifiers.invoiceNumber {
            text = currentItem.invoiceNumber!
            cellIdentifier = CellIdentifiers.invoiceNumberCell
        }
        if currentColumn == fieldIdentifiers.invoiceDate {
            text  = currentItem.invoiceDate!
            cellIdentifier = CellIdentifiers.invoiceDateCell
        }
        if currentColumn == fieldIdentifiers.invoiceDueDate {
            text  = currentItem.invoiceDueDate!
            cellIdentifier = CellIdentifiers.invoiceDueDateCell
        }
        if currentColumn == fieldIdentifiers.invoiceTotal {
            text  = currentItem.invoiceTotal!
            cellIdentifier = CellIdentifiers.invoiceTotalCell
        }
        if currentColumn == fieldIdentifiers.invoiceAmountDue {
            text  = currentItem.invoiceAmountDue!
            cellIdentifier = CellIdentifiers.invoiceAmountDueCell
        }
        if currentColumn == fieldIdentifiers.invoiceAppliedAmount {
            text  = currentItem.invoiceAppliedAmount!
            cellIdentifier = CellIdentifiers.invoiceAppliedAmountCell
        }
        if let cell = tableView.makeView(withIdentifier: NSUserInterfaceItemIdentifier(rawValue: cellIdentifier), owner: self) as? NSTableCellView {
            cell.textField?.stringValue = text
            return cell
        }
        return nil
    }

I would like the amount cells to be red if they are less than zero and green for => zero. How is this down within this function?