OnDraw routine doesn't work as expected

I wish to color my DBGrid as for the following schema:
- All of the DBGrid with black cahracters on a white background
- If the user goes into edit mode, I would like the cell the user wants to

edit to get a yellow background and the entire ROW a grey background. The 
cell should keep its yellow background while the user is typing.
I have written the following procedure to realize what above, but 
unfortunately it doesn't work as expected: in fact when entering the edit 
mode (clicking a button):
- the ROW doesn't get its grey background,
- the yellow background is on the first cell of the row (and this may be 
right), but as soon as one starts typing, the background turns to white..

Can you understand why or give me a suggestion ?

  with Sender as TDBGrid do
  begin
    if dgRowSelect in Options then
       // dgRowSelect ON = DBGrid in Browse mode
       DefaultDrawColumnCell(Rect, Datacol, Column, State)
    else
    begin
       // dgRowSelect OFF = DBGrid in Edit mode
       if State = [gdFocused, gdSelected] then
       begin
          //  yellow background on the cell in edit
          Canvas.Brush.Color := clYellow;
          Canvas.Font.Color := clBlack;
       end else
       begin
          //  grey background on the row of the cell in edit
          Canvas.Brush.Color := clGrey;
          Canvas.Font.Color := clBlack;
       end;
        DefaultDrawColumnCell(Rect, Datacol, Column, State);
    end;
  end;