// in FormCreate:

with ComboBox1.Items do 

begin 

  Add(IntToStr(clRed)); 

  Add(IntToStr(clFuchsia)); 

  Add(IntToStr(clBlue)); 

  Add(IntToStr(clGreen));

  Add(IntToStr(clYellow));

end;

procedure TForm1.ComboBox1DrawItem(Control: TWinControl;

  Index : Integer; Rect: TRect; State: TOwnerDrawState);

begin

  with Control as TComboBox,Canvas do 

  begin 

    // fill the rectangle first with white

    Brush.Color := clWhite; 

    FillRect(Rect); 

    // then reduce it and fill it with the color

    InflateRect(Rect,-2,-2); 

    Brush.Color := StrToInt(Items[Index]); 

    FillRect(Rect);

 end;

end;