I have an invoice from December that was paid in January. Now, in the Customer Ledger the payment is in black, but the invoice is still in red?
Red + Bold + Italics indicates that the Invoice has crossed it due date and is still unpaid.
Red + Italics indicates that the payment has been applied but after the due date.
So, even if the invoice is closed, it will show in Red if the payment is applied after the invoice due date. But, it will not be shown in BOLD as bold indicates "Open Invoices" with due date crossed.