Почему средство выбора в CollectionView устанавливает для привязки SelectedItem значение null?

Вы можете поместить переменную ColorsPicker и переменную ColorNav внутри модели Player.cs и реализовать интерфейс INotifyPropertyChanged для модели Player.cs.

public class Player: INotifyPropertyChanged
{
    public string Name { get; set; } = default!;
    public int ColorId { get; set; } = default!;
    // public Colour ColorNav { get; set; } = default!;

    public ObservableCollection<Colour> ColorsPicker { get; set; } = new ObservableCollection<Colour>();


    //add a property for the SelectedItem property of picker
    private Colour _colorNav;
    public Colour ColorNav
    {
        set
        {
            SetProperty(ref _colorNav, value);
        }
        get { return _colorNav; }
    }

    bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
    {
        if (Object.Equals(storage, value))
            return false;
        storage = value;
        OnPropertyChanged(propertyName);
        return true;
    }
    protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
    public event PropertyChangedEventHandler PropertyChanged;
}

Кроме того, если мы хотим обновить пользовательский интерфейс (<Label Grid.Column = "2" Text = "{Binding ColorNav.Name}") при изменении выбранного элемента текущего средства выбора, нам также необходимо реализовать интерфейс INotifyPropertyChanged для модели Colour.cs

public class Colour: INotifyPropertyChanged
{
    public int ColorId { get; set; }
    //public string Name { get; set; } = default!;

    private string _name;
    public string Name
    {
        set
        {
            SetProperty(ref _name, value);
        }
        get { return _name; }
    }
    bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
    {
        if (Object.Equals(storage, value))
            return false;
        storage = value;
        OnPropertyChanged(propertyName);
        return true;
    }
    protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
    public event PropertyChangedEventHandler PropertyChanged;

}

Затем мы можем инициализировать MainViewModel.cs следующим образом:

public class MainViewModel
{
    public ObservableCollection<Colour> ColorsPicker { get; set; } = new ObservableCollection<Colour>();
    public ObservableCollection<Player> PlayerList { get; set; } = new ObservableCollection<Player>();

    public MainViewModel() {

        ColorsPicker = [
     new Colour {
                ColorId = 1,
                Name = "Red"
            },
            new Colour {
                ColorId = 2,
                Name = "Blue"
            },
            new Colour {
                ColorId = 3,
                Name = "Yellow"
            },
        ];

        PlayerList = [
            new Player {
                Name = "John",
                ColorId = 1,
                ColorNav = ColorsPicker[0],
                ColorsPicker = ColorsPicker // initialize ColorsPicker
            },
            new Player {
                Name = "Carla",
                ColorId = 3,
                ColorNav = ColorsPicker[2],
                ColorsPicker = ColorsPicker // initialize ColorsPicker

            },
        ];

    }
}

И затем мы можем связать данные следующим образом:

    <CollectionView ItemsSource = "{Binding PlayerList}">
        <CollectionView.ItemTemplate>
            <DataTemplate x:DataType = "model:Player">
                <SwipeView>
                    <Border>
                        <Grid ColumnDefinitions = "*, *, *" ColumnSpacing = "1" >
                            <Label Grid.Column = "0" Text = "{Binding Name}" />
                            <!--<Picker Grid.Column = "1" ItemsSource = "{Binding ColorsPicker, Source = {RelativeSource AncestorType = {x:Type viewmodel:MainViewModel}}}" ItemDisplayBinding = "{Binding Name}" SelectedItem = "{Binding ColorNav}"/>-->
                            <Picker Grid.Column = "1" ItemsSource = "{Binding ColorsPicker}" ItemDisplayBinding = "{Binding Name}" SelectedItem = "{Binding ColorNav ,Mode=TwoWay}"/>

                            <Label Grid.Column = "2" Text = "{Binding ColorNav.Name}"/>
                        </Grid>
                    </Border>
                </SwipeView>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>