A:  Assign a procedure to the screen.OnActiveControlChange event.

unit Killer2;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  Forms, Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Edit2: TEdit;
    Edit3: TEdit;
    Edit4: TEdit;
    Edit5: TEdit;
    Edit6: TEdit;
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure DoActiveControl(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  OldControl: TComponent;

implementation

{$R *.DFM}

procedure TForm1.FormCreate(Sender: TObject);
begin
  screen.OnActiveControlChange := DoActiveControl;
end;

procedure TForm1.DoActiveControl(Sender: TObject);
begin
{This goes first in case the active control is not a TEdit.}
  if assigned(OldControl) then
  begin
    (OldControl as TEdit).color := clWhite;
    (OldControl as TEdit).font.color := clBlack;
  end;
  if activeControl is TEdit then
  begin
    (activeControl as TEdit).color := clNavy;
    (activeControl as TEdit).font.color := clYellow;
    OldControl := activeControl as TEdit;
  end;
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  screen.OnActiveControlChange := nil; {prevents a GPFault}
end;

end.