class ViewModelOrbs : ViewModelBase
    {
        ObservableCollection<Orb> _orbs;
        object _selectedOrb;

        public RelayCommand AddCommand { get; set; }
        void AddOrb(object parameter)
        {
            SelectedOrb = null;
            var orb = new Orb();
            _orbs.Add(orb);
            SelectedOrb = orb;
            RaisePropertyChanged("Orbs");
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            AddOrb dtd = new AddOrb();
            dtd.Owner = this;
            dtd.WindowStartupLocation = WindowStartupLocation.CenterOwner;
            var orb = new Orb();
            orb.Color = "#000000";
            dtd.DataContext = orb;
            dtd.Show();
        }