How to randomly fire at a 10 by 10 grid?

The following is adapted from a WPF implementation of the 2048 game. I used a UniformGrid and filled it with Buttons. The buttons were styled to appear as blank coloured tiles. A converter was used to convert the Button.Tag property value to a Fill colour so that the colour of the tile could be changed by simply changing the Tag value.

public partial class MainWindow : Window
    {
        private readonly List<Button> tiles = new();
        private Random random = new();
        private int lastSelectedIndex;
        public MainWindow()
        {
            InitializeComponent();
          //  var viewModel = new GameViewModel();
          //  viewModel.TilesUpdatedEvent += TilesUpdatedEventhandler;
            AddEmptyTilesToCollection();
            lastSelectedIndex = 0;
          //  DataContext = viewModel;
          //  viewModel.StartGame();
        }

        private void AddEmptyTilesToCollection()
        {
            var style = (Style)FindResource("TileButton");

            for (int id = 0; id < 16; id++)
            {
                var tile = new Button
                {
                    Style = style,
                    Tag = 0                 
                  
                };
                tile.Click += OnTileClick;
                tiles.Add(tile);
                Board.Children.Add(tile);
            }

        }
        private void OnTileClick(object sender, RoutedEventArgs e)
        {
            int index = random.Next(tiles.Count);
            var selectedTile = tiles[index];
            tiles[lastSelectedIndex].Tag = 0;
            selectedTile.Tag = 1;
            lastSelectedIndex = index;
        }
    }

<Window x:Class="ColouredTiles.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:ColouredTiles"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <UniformGrid Margin="2" Grid.Row="0" Rows="4" Columns="4"  Name="Board" Background="White"  Width="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}" >

        </UniformGrid>
    </Grid>
</Window>

<Application x:Class="ColouredTiles.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:ColouredTiles"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <local:IntToBrushConverter x:Key="intToBrushConverter"/>
        <Style x:Key="TileButton" TargetType="Button">
            <Setter Property="OverridesDefaultStyle" Value="True"/>
            <Setter Property="Margin" Value="5"/>
            <Setter Property="Background" Value="LightGray"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="Button">
                        <Grid >
                            <Rectangle Fill="{TemplateBinding Tag,Converter={StaticResource intToBrushConverter}}"
                                         Stroke="{TemplateBinding BorderBrush}"
                                         StrokeThickness="6" 
                                Height="100"
                                       Width="100"     
                                         />
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Application.Resources>
</Application>

public class IntToBrushConverter : IValueConverter
   {//always check for null as this may be called at startup with a null value
       public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
       {
           if (value == null)
           {
               return new SolidColorBrush();//colourless
           }

           int index = (int)value;
           return index > tileColours.Length - 1 ? new SolidColorBrush() : new SolidColorBrush(tileColours[index]);
       }

       public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
       {
           throw new NotImplementedException();
       }
       private readonly Color[] tileColours =
  {
           Colors.LightGray,     // 0
           Colors.Yellow,        // 1
           Colors.Red,           // 2
           Colors.Green,         // 3
           Colors.Orange,        // 4
           Colors.LightBlue,     // 5
           Colors.Magenta,       // 6
           Colors.LightPink,     // 7
           Colors.Goldenrod,     // 8
           Colors.Aquamarine,    // 9
           Colors.Blue,          // 10
            Colors.LightGreen,   // 11
           Colors.DarkRed,       // 12
           Colors.DarkBlue,      // 13
           Colors.DarkOrange,    // 14
           Colors.Black          // 15
       };


   }