Автор: http://www.swissdelphicenter.ch

function DesktopColor(const X, Y: Integer): TColor;
 var
   c: TCanvas;
 begin
   c := TCanvas.Create;
   try
     c.Handle := GetWindowDC(GetDesktopWindow);
     Result   := GetPixel(c.Handle, X, Y);
   finally
     c.Free;
   end;
 end;

 procedure TForm1.Timer1Timer(Sender: TObject);
 var
   Pos: TPoint;
 begin
   GetCursorPos(Pos);
   Panel1.Color := DesktopColor(Pos.X, Pos.Y);
 end;