procedure TForm1.FormCreate(Sender: TObject);
begin
    Form1.Color := clBlack;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin              //This example will show one pixel coloured Yellow
    Canvas.Pixels[100, 100] := clYellow;
end;
Make sure the button is at the bottom of the form, or you might not see the pixel.