procedure TForm1.DBGrid1DrawDataCell(Sender: TObject; const 
  Rect: TRect; Field: TField; State: TGridDrawState);
begin
  if Field.FieldName = 'SERIAL' then
    if (Field as TStringField).Value = 'НЕИЗВЕСТНО' then
      with (Sender as TDBGrid).Canvas do 
      begin
        Brush.Color := clRed;
        Font.Style := [fsItalic];
        Font.Color := clAqua;
        FillRect(Rect);
        TextOut(Rect.Left, Rect.Top, Field.AsString);
      end;
end;