[NumLock]

нужно обрабатывать WM_CTLCOLOREDIT:

Код:

function WindowProc(hwnd: HWND; Msg: Integer;
  wParam: WPARAM; lParam: LPARAM): Longint; stdcall;
begin
  case Msg of
    WM_CTLCOLOREDIT: begin
      SetBkColor(wParam, $00);
      SetTextColor(wParam, $ffffff);
      Result:=GetStockObject(BLACK_BRUSH);
    end;