BeatDetector/MainWindow.xaml

<Window x:Class="BeatDetector.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:BeatDetector" d:DataContext="{d:DesignInstance Type=local:MainWindow}"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800" KeyUp="Window_KeyUp" ResizeMode="NoResize">
    <Grid Name="MainGrid">
        <ItemsControl ItemsSource="{Binding BeatLights}" Background="Black">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Grid Background="{Binding Color}" Margin="5" Width="100" Height="100"></Grid>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <WrapPanel />
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
        </ItemsControl>
    </Grid>
</Window>