Csharp 如何绑定到分组列表中对象内的属性值

首先，感谢你对我上一个问题here的解答

该解决方案效果很好，但是，我正在努力绑定其中一个属性的更改.当用户界面加载以下代码时，我的背景 colored颜色 绑定良好:

<StackLayout x:Name="stlShippingOrders" Spacing="5" Padding="0,0,0,5" >
        <CollectionView Margin="0,-6,0,0" IsGrouped="True" ItemsSource="{Binding 
ShippingItems}">
            <CollectionView.GroupHeaderTemplate>
                <DataTemplate>
                    <StackLayout Padding="5,-5,5,-5" Orientation="Horizontal" 
BackgroundColor="LightBlue" HeightRequest="45">
                        <Label Text="{Binding Shipment.OrderCode}" 
VerticalOptions="Center"/>
                        <Label Text="{Binding Shipment.CustomerName}" 
VerticalOptions="Center"/>
                        <Label Text="{Binding Shipment.DeliveryDate}" 
HorizontalOptions="EndAndExpand" VerticalOptions="Center" />
                    </StackLayout>
                </DataTemplate>
            </CollectionView.GroupHeaderTemplate>
            <CollectionView.ItemTemplate>
                <DataTemplate>
                    <Label Padding="5,2,5,2" Text="{Binding ItemDescription}" 
BackgroundColor="{Binding BackgroundColour}"/>
                </DataTemplate>
            </CollectionView.ItemTemplate>
        </CollectionView>
    </StackLayout>

ShippingItem Model (with added BacgroundColour property)

 public class ShippingItem
{
    public String ShippingItemId { get; set; }
    public string ShippingItemScanCode { get; set; }
    public string ItemDescription { get; set; }
    public string ShippingOrderId { get; set; }
    public bool IsLoaded { get; set; }

    // Default set to transparent #00FFFFFF
    public string BackgroundColour { get; set; }  = "#00FFFFFF";

}

Class for grouping

public class ShippingItemGroup : ObservableCollection<ShippingItem>
{
    public ShippingOrder Shipment { get; private set; }

    public ShippingItemGroup(ShippingOrder shipment, ObservableCollection<ShippingItem> 
shippingItems) : base(shippingItems)
    {
        Shipment = shipment;
    }
}

ViewModel code that creates the group list and binds correctly

                        public ObservableCollection<ShippingItemGroup> ShippingItems { 
get; private set; } = new ObservableCollection<ShippingItemGroup>();
                    foreach (var item in thisShipment.ShippingOrders)
                    {
                        ShippingItems.Add(new ShippingItemGroup(item, 
item.ShippingItems));
                    }

ViewModel code that updates the list object when it is loaded. This is the bit i'm having trouble with, the shippingItem.BackgroundColour = "FFA533" changes the collection but doesn't update the UI.

foreach (ShippingItemGroup shippingItemGroup in ShippingItems)
                    {
                        foreach(ShippingItem shippingItem in 
shippingItemGroup.Shipment.ShippingItems)
                        {
                            if(shippingItem.ShippingItemScanCode == scanCode)
                            {
                                shippingItem.IsLoaded = true;
                                shippingItem.BackgroundColour = "FFA533";
                                IsShipmentHeaderVisible = true;
                                IsShippingOrdersVisible = true;
                                IsErrorMessageVisible = false;
                                ScanCode = "";
                            }
                        }
                    }