Draw a color outline around TForm

Create a message handler for the Windows Message WM_NCPAINT
message. The following example paints a 1 pixel red border around the
frame of the form.

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
  private
         procedure WMNCPaint(var Msg: TWMNCPaint); message WM_NCPAINT;
  public
        //
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.WMNCPaint(var Msg: TWMNCPaint);
var
  dc: hDc;
  Pen: hPen;
  OldPen: hPen;
  OldBrush: hBrush;
begin
  inherited;
  
  dc := GetWindowDC(Handle);
  Msg.Result := 1;

  //Change the RGB value to change the color
  Pen := CreatePen(PS_SOLID, 1, RGB(255, 0, 0));
  OldPen := SelectObject(dc, Pen);
  OldBrush := SelectObject(dc, GetStockObject(NULL_BRUSH));
  Rectangle(dc, 0, 0, Form1.Width, Form1.Height);
  SelectObject(dc, OldBrush);
  SelectObject(dc, OldPen);
  DeleteObject(Pen);
  ReleaseDC(Handle, Canvas.Handle);
end;

end.