Xamarin XAML Binding to an iMarkupExtension

Simple Proof of Concept (I would use a Converter for this, but am learning Markup Extensions)

This is based on the Evolve 2016 session https://evolve.xamarin.com/session/56e201d2bad314273ca4d813

I have a page bound to a simple model with a Label.

        <Label 
            Text="{Binding Path=Amount1, StringFormat='{0:C}'}" 
            TextColor="{toolkit:FinancialColor PrettyMoney={Binding Amount1}}"></Label>

I need to send the Amount1 from the Model to the IMarkupExtenion FinancialColor, but I get an error that PrettyMoney has no property, bindable property or event.

Here's my MarkupExtension

public class FinancialColorExtension : IMarkupExtension
{
    public decimal PrettyMoney { get; set; }

    public object ProvideValue(IServiceProvider serviceProvider)
    {
        if (PrettyMoney == 0) return Color.Black;
        if (PrettyMoney < 0)  return Color.Red;
        return Color.Green;
    }
}