Navigating from one screen to another in Caliburn - wpf

Question

I am using a sample from http://www.mindscapehq.com/blog/index.php/2013/09/11/caliburn-micro-part-6-introduction-to-screens-and-conductors/
There is an AppViewModel in which other ViewModels are activated by calling ActivateItem.
The sample is working for me: I can see the corresponding View.
I now want to activate a ViewModel from another ViewModel. It gets instantiated but the corresponding View is not displayed.
How can I activate "GreenScreenViewModel" from "RedScreenViewModel"?
AppView:
<UserControl x:Class="CaliburnMicroApp_Navigation.AppView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">
<DockPanel Background="LightBlue" MinHeight="400" MinWidth="400">
<StackPanel Orientation="Horizontal" DockPanel.Dock="Top" HorizontalAlignment="Center">
<Button x:Name="ShowRedScreen" Content="Red" Width="50" />
<Button x:Name="ShowGreenScreen" Content="Green" Width="50" Margin="12,0,0,0" />
<Button x:Name="ShowBlueScreen" Content="Blue" Width="50" Margin="12,0,0,0" />
</StackPanel>
<ContentControl x:Name="ActiveItem" />
</DockPanel>
</UserControl>
AppViewModel
public class AppViewModel : Conductor<object>
{
public void ShowRedScreen()
{
ActivateItem(new RedViewModel());
}
public void ShowGreenScreen()
{
ActivateItem(new GreenViewModel());
}
public void ShowBlueScreen()
{
ActivateItem(new BlueViewModel());
}
}
RedViewModel - this does not display GreenView ()
public class RedViewModel : Conductor<object>
{
public void DisplayGreen()
{
ActivateItem(new GrenViewModel());
}
}
RedView
<Grid Background="Red">
<StackPanel>
<TextBlock Text="red" FontSize="48" FontWeight="Bold" Foreground="#3CA527" />
<Button Name="DisplayGreen">
<TextBlock >Next Screen</TextBlock>
</Button>
</StackPanel>
</Grid>

Answer 1

If you want the ActiveItem displayed in the ContentControl of your AppView to change when you press your Button in RedViewModel, you will need to use the EventAggregator to pass a message from your RedViewModel to your AppViewModel.
Mindscape EventAggregator tutorial