procedure TForm1.Button1Click(Sender: TObject);
var
  col: TColor;
  r, g, b: integer;
  x, y: integer;
  can: TCanvas;
begin
  can := TCanvas.Create;
  can.Handle := GetDC(0);
  for x := 0 to 100 do begin
    for y := 0 to 100 do begin
      col := Can.Pixels[x,y];
      r := GetRValue(col) - 30;
      if r < 0 then r := r + 256;
      g := GetGValue(col) - 20;
      if g < 0 then g := g + 256;
      b := GetBValue(col) - 10;
      if b < 0 then b := b + 256;
      Can.Pixels[x,y] := RGB(r, g, b);
    end;
  end;
  ReleaseDC(0, can.Handle);
  can.Destroy;
end;