UWP ComboBox SelectedValue not being set

I obviously don't grasp using SelectedValue to change which item a combo box is showing in UWP

The XAML is simple

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <ComboBox x:Name="comboBoxColor" />
</Grid>

The code-behind isn't very deep

    public MainPage()
    {
        this.InitializeComponent();
    }

    public void page_Loaded(object sender, RoutedEventArgs e)
    {
        populateDdlMultiColor();

        comboBoxColor.SelectedValue = Colors.Red;
        //comboBoxColor.SelectedIndex = 0 works tho
    }

    private void populateDdlMultiColor()
    {
        comboBoxColor.ItemsSource = ColorDict();
        comboBoxColor.DisplayMemberPath = "Key";
        comboBoxColor.SelectedValuePath = "Value";


    }

    private Dictionary<string,Color> ColorDict()
    {
        Dictionary<string, Color> L = new Dictionary<string, Color>();
        L.Add("reddish",Colors.Red);

        return L;
    }

This is obviously tinker-toy but it fails the same way my code fails: After setting the SelectedValue, the combo box is on index -1 and SelectedValue is null. If I set SelectedIndex to a proper value [see comment] the combo box works - it has been loaded.