procedure TformMain.ReplaceColor(aSourceColor, aTargetColor: TColor; AHueTolerance, ASatTolerance, ALumTolerance: Integer);
begin
  for each Pixel in AllImagePixels do
  begin
    if ((Pixel.Hue - aSourceColor.Hue) < AHueTolerance) and ((Pixel.Sat - aSourceColor.Sat) < ASatTolerance) and ((Pixel.Lum - aSourceColor.Lum) < ALumTolerance) then
      Pixel.Color := aTargetColor;
  end;
end;

procedure TForm1.btnTestClick(Sender: TObject);
const
  T = 10; // tolerance value
var
  rgb: TRGB;
begin
  rgb := TColor2TRGB( $00FEFE ); // this is a color in the image
  imgTest.Proc.CastColorRange(
    TRGB2TColor(CreateRGB(rgb.r + T, rgb.g + T, rgb.b + T)), // BeginColor
    TRGB2TColor(CreateRGB(rgb.r - T, rgb.g - T, rgb.b - T)), // EndColor
    clRed); // CastColor
end;

procedure TForm1.btnTestClick(Sender: TObject);
const
  T = 10; // tolerance value
var
  H, S, V: Integer;
begin
  ColorToHSV($0001FD, H, S, V); // this is a color in the image

  imgTest.Proc.CastColorRange(
    HSVToColor(H, S + T, V - T), // BeginColor
    HSVToColor(H, S - T, V + T), // EndColor
    clBlue); // CastColor
end;