Cannot get visual appearance of a UserControl to change

I use a UserControl called Dot to draw circles on a map:

<UserControl
    x:Class="Dot"
    .....
    <Ellipse
        Fill="{Binding Path=FillColor}"
        Height="{Binding Path=Diameter}"
        Width="{Binding Path=Diameter}">
    </Ellipse>
</UserControl>

This is how I draw it:

Private Sub DrawPoint(Colr As SolidColorBrush, Radius As Double)
    ADot = New Dot With {.FillColor = Colr, .Diameter = 2 * Radius}
    MapLayr.AddChild(ADot, PointLocation, PositionOrigin.Center)
End Sub

Initially the colr is Red. After validation I want to change the color to green using:

ADot.FillColor = Brushes.Green

That does not change the appearance of the dot on the map. What more needs to be done to actually change the visible color?