grayscale MDIChild form when it becomes inactive

for all Child forms:

procedure TForm2.FormActivate(Sender: TObject);
begin
  Color := clBtnFace;
end;

procedure TForm2.FormDeactivate(Sender: TObject);
begin
  Color := clInactiveBorder
end;

emil

procedure TForm2.FormActivate(Sender: TObject);
begin
  Color := clBtnFace;
  Memo1.Color := clWindow;
  Memo1.Font.Color := clWindowText;
end;

procedure TForm2.FormDeactivate(Sender: TObject);
begin
  Color := clInactiveBorder;
  Memo1.Color := clScrollBar;
  Memo1.Font.Color := clGrayText;
end;

This is not what I meant:  

What is needed is that all the form appears in grayscale (even images on it) as if a colored photo is turned into grayscale.

Take a look at:
https://www.experts-exchange.com/questions/20075307/converting-a-BMP-image-to-gray-scale.html
and (or)
https://www.experts-exchange.com/questions/20545410/How-to-use-Scanline-property-of-bitmap-to-read-an-entire-line-of-pixels-into-a-2d-array.html

It could be something like this:

unit Unit1;

interface

uses
  Windows, Classes, Forms, Graphics;

type
  TForm1 = class(TForm)
    procedure ApplicationEventsMessage(var Msg: tagMSG; var Handled: Boolean);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  GrayBMP : TBitmap;

implementation

{$R *.dfm}

procedure TForm1.ApplicationEventsMessage(var Msg: tagMSG; var Handled: Boolean);
type
  TLine = array [0..32767] of TRGBTriple;
  PLine = ^TLine;
var
  l : PLine;
  w, h : integer;
begin
if msg.message = 45057 then begin
  GrayBMP := Form1.GetFormImage;
  GrayBMP.PixelFormat := pf24bit;
  for h := 0 to GrayBMP.Height - 1 do begin
    l := GrayBMP.ScanLine[h];
    for w := 0 to GrayBMP.Width - 1 do with l[w] do begin
      rgbtRed := (rgbtRed + rgbtGreen + rgbtBlue) div 3;
      rgbtGreen := rgbtRed;
      rgbtBlue := rgbtRed;
    end;
  end;
  msg.message := 15;
end else if msg.message = 45056 then Form1.Refresh;
if (Msg.message = 15) and not Form1.Active then Canvas.Draw(0,0,GrayBMP);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
Application.OnMessage := ApplicationEventsMessage;
end;

unit Unit_Child;

interface

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

const
  First_Child = 1;
  Last_Child = 8;

type
  TPaintMode = (pmColorful, pmGrayScale);
  TForm2 = class(TForm)
    Memo1: TMemo;
    Image1: TImage;
    procedure FormActivate(Sender: TObject);
    procedure FormDeactivate(Sender: TObject);
    procedure FormPaint(Sender: TObject);
  private { Private declarations }
    PaintMode:TPaintMode;
  public  { Public declarations }
  end;

var
  Form_2_arr: array[First_Child..Last_Child] of TForm2;

implementation

{$R *.dfm}

procedure TForm2.FormActivate(Sender: TObject);
var
  I:      Integer;
begin
  PaintMode := pmColorful;
  with Parent do
    for I := MDIChildCount-1 downto 0 do
      MDIChildren[I].Repaint;
  for I := 0 to ComponentCount-1 do
    if (Components[I] is TWinControl) then
      (Components[I] as TWinControl).Repaint;
  Repaint;
end;

procedure TForm2.FormDeactivate(Sender: TObject);
var
  I:      Integer;
begin
  PaintMode := pmGrayScale;
  with Parent do
    for I := MDIChildCount-1 downto 0 do
      MDIChildren[I].Repaint;
  Repaint;
end;

procedure TForm2.FormPaint(Sender: TObject);
var
  FmDc:   HDC;
begin
  if (PaintMode=pmGrayScale) then
  begin
    FmDc := GetWindowDC(Self.Handle);
    SelectObject(FmDc, GetStockObject(GRAY_BRUSH));
    PatBlt(FmDc, 0, 0, Width,Height,$A000C9);
    ReleaseDC(Self.Handle, FmDc);
  end;
end;

var
  I:      Integer;
initialization

for I := First_Child to Last_Child do
  Form_2_arr[I] := nil;

end.

The solution of esoftbg darkens the form but does not convert it to grayscale

The solution of ZhaawZ seems to be a good one BUT: the controls are being re-drawn over their grayscale image, thus effect is lost.  The only way I could decide it was the beginning of good solution is by changing the Canvas.Draw(0,0,GrayBMP) to Canvas.Draw(10,10,GrayBMP) (i.e shifted) and see that controls are being painted in grayscale,  but then overriden by the redraw of real controls themselves.

so ZhaawZ, any solution for that ?

This should do... thanks
I will accept this as a good answer

My last version is at             http://www.geocities.com/esoftbg
                        file             Q_20944061.exe.zip
emil

ooops,
i am late