Code:
    procedure TForm1.FormPaint(Sender: TObject);
    var
    Row, Ht: Word ;
    begin
    Ht := (ClientHeight + 255) div 256 ;
    for Row := 0 to 255 do
    with Canvas do begin
    Brush.Color := RGB(Row, 0{Row}, {Row}0) ;
    FillRect(Rect(0, Row * Ht, ClientWidth, (Row + 1) * Ht)) ;
    end ;
    end;