var
  Form1: TForm1;
  blueVal: Byte;
  BluePalette: HPalette;
implementation

{$R *.DFM}

procedure TForm1.FormCreate(Sender: TObject);
var

  LogicalPalette: PLogPalette;
  ColorIndex: LongInt;
begin
  GetMem(LogicalPalette, (SizeOf(TLogPalette) + SizeOf(TPaletteEntry) * 256));
  GetSystemPaletteEntries(Canvas.Handle, 0, 256,
    LogicalPalette^.palPalEntry[0]);
  with LogicalPalette^ do

  begin
    palVersion := $300;
    palNumEntries := 256;
{$R-}
    for ColorIndex := 10 to 245 do
      with palPalEntry[ColorIndex] do
      begin
        peRed := 0;
        peGreen := 0;
        peBlue := 255 - (ColorIndex - 10);
        peFlags := PC_NOCOLLAPSE;
      end;
  end;
{$R+}
  BluePalette := CreatePalette(LogicalPalette^);
  FreeMem(LogicalPalette, (SizeOf(TLogPalette) + SizeOf(TPaletteEntry) * 256));
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin

  DeleteObject(BluePalette);
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;

  Shift: TShiftState; X, Y: Integer);
var

  OldPal: HPALETTE;
begin

  OldPal := SelectPalette(Canvas.Handle, BluePalette, False);
  RealizePalette(Canvas.Handle);
  canvas.pen.color := $02000000 or (BlueVal * $00010000);
  canvas.pen.width := 10;
  canvas.moveto(0, 0);
  canvas.lineto(X, Y);
  SelectPalette(Canvas.Handle, OldPal, False);
  Inc(BlueVal);

  if BlueVal > 255 then
    BlueVal := 0;
end;