How can I replace color on TCanvas on Delphi?

Question

How can I replace color on TCanvas on Delphi XE2? The following code works incredibly slow:

  for y := ARect.Top to ARect.Top + ARect.Height - 1 do
    for x := ARect.Left to ARect.Left + ARect.Width - 1 do
      if Canvas.Pixels[x, y] = FixedColor then
        Canvas.Pixels[x, y] := Canvas.Pixels[ARect.Left, ARect.Top];

Answer 1

var
  aBitmap: TBitmap;
  x, y: Integer;
  aPixel: PRGBTriple;

 ...

  aBitmap := TBitmap.Create;
  try
    aBitmap.PixelFormat := pf24bit;
    aBitmap.Height := ARect.Height;
    aBitmap.Width := ARect.Width;
    aBitmap.Canvas.CopyRect(TRect.Create(0, 0, aBitmap.Width, aBitmap.Height), Canvas, ARect);
    for y := 0 to aBitmap.Height - 1 do
      for x := 0 to aBitmap.Width - 1 do
      begin
        aPixel := aBitmap.ScanLine[y];
        Inc(aPixel, x);
        if (aPixel^.rgbtRed = GetRValue(FixedColor)) and (aPixel^.rgbtGreen = GetGValue(FixedColor)) and (aPixel^.rgbtBlue = GetBValue(FixedColor)) then
          aPixel^ := PRGBTriple(aBitmap.ScanLine[y])^;
      end;
    Canvas.Draw(ARect.Left, ARect.Top, aBitmap);
  finally
    aBitmap.Free;
  end;