How can I access my ViewModel from code behind

Quick answer. This might help others as well

((MyViewModel)(this.DataContext)).MyProperty

In MVVM you shouldn't be accessing your view model from code behind, the view model and view are ignorant of each other a here endeth the lecture :)

Instead you can attach the EventToCommand behaviour to your control. This lets you bind an event in the control to a command in the data context. See msdn commands tutorial here.

If you are desperate to do it, you can access the controls data context property and cast it to your view model type to give access to the internals.

var vm = (ViewModelType)this.DataContext;
vm.CommandProperty.Execute(null);

This isn't too difficult. First, create an instance of your ViewModel inside your Window XAML:

View XAML:

<Window x:Class="BuildAssistantUI.BuildAssistantWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:VM="clr-namespace:MySolutiom.ViewModels">
     <Window.DataContext>
         <VM:MainViewModel />
     </Window.DataContext>
  </Window>

After that, you can System.Windows.Interactivity.InvokeCommandAction to translate your event to a command:

View XAML:

<Grid>
 <Rectangle x:Name="Color01" Fill="#FFF4F4F5" HorizontalAlignment="Left" Height="100" Margin="10,29,0,0" Stroke="Black" VerticalAlignment="Top" Width="100" MouseDown="Color_MouseDown">
   <interactivity:Interaction.Triggers>
      <interactivity:EventTrigger EventName="MouseDown">
          <interactivity:InvokeCommandAction Command="{Binding MyCommand}"/>
      </interactivity:EventTrigger>
   </interactivity:Interaction.Triggers>
 </Rectangle>
</Grid>

Now, in your ViewModel, set up an ICommand and the DelegateCommand implementation to bind to that event:

ViewModel:

public class ViewModel
{
    public ICommand MyCommand { get; set; }

    public ViewModel()
    {
        MyCommand = new DelegateCommand(OnRectangleClicked);
    }

    public void OnRectangleClicked()
    {
        // Change boolean here
    }
}