<ModelVisual3D x:Name="Model3D">
<ModelVisual3D.Content>
<Model3DGroup x:Name="Group">
<AmbientLight Color="DarkCyan" />
<DirectionalLight Color="White" Direction="-5,-8,-7" />
</Model3DGroup>
</ModelVisual3D.Content>
</ModelVisual3D>
Group.Children.Add()
Group.Children.Clear()
Group.Children.Add(new ModelVisual3D() { Content = new AmbientLight(Colors.White) });