Вы ищете что-то вроде этого:
<Page
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d">
<Grid x:Name="gdStar" Width="Auto" Height="Auto">
<Path
Fill="Red" Stretch="Fill" Stroke="Blue" StrokeThickness="1" Margin="-0.5,-0.5,-0.5,-0.019" Data="M63.50031,-0.50054431 L47.500523,55.499079 -0.49883747,55.499079 39.50063,95.498578 23.500843,159.49799 63.833676,128.4989 103.7623,160.51698 87.095797,95.850405 127.49946,55.499079 79.500097,55.499079 z">
<Path.OpacityMask>
<LinearGradientBrush StartPoint="0,0" EndPoint="1,0">
<GradientStop Color="Transparent" Offset="0.3"/>
<GradientStop Color="White" Offset="0.3"/>
</LinearGradientBrush>
</Path.OpacityMask>
</Path>
<Path
Fill="Transparent" Stretch="Fill" Stroke="Red" StrokeThickness="1" Margin="-0.5,-0.5,-0.5,-0.019" Data="M63.50031,-0.50054431 L47.500523,55.499079 -0.49883747,55.499079 39.50063,95.498578 23.500843,159.49799 63.833676,128.4989 103.7623,160.51698 87.095797,95.850405 127.49946,55.499079 79.500097,55.499079 z"/>
</Grid>
</Page>
Редактировать Вот версия, которая изменяет фон незаполненной области, добавляя за ней белую звезду, я оставил предыдущий пример, потому что она немного понятнее.
<Page
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" Background="yellow">
<Grid x:Name="gdStar" Width="Auto" Height="Auto">
<Path
Fill="White" Stretch="Fill" Stroke="Blue" StrokeThickness="1" Margin="-0.5,-0.5,-0.5,-0.019" Data="M63.50031,-0.50054431 L47.500523,55.499079 -0.49883747,55.499079 39.50063,95.498578 23.500843,159.49799 63.833676,128.4989 103.7623,160.51698 87.095797,95.850405 127.49946,55.499079 79.500097,55.499079 z"/>
<Path
Fill="Red" Stretch="Fill" Stroke="Blue" StrokeThickness="1" Margin="-0.5,-0.5,-0.5,-0.019" Data="M63.50031,-0.50054431 L47.500523,55.499079 -0.49883747,55.499079 39.50063,95.498578 23.500843,159.49799 63.833676,128.4989 103.7623,160.51698 87.095797,95.850405 127.49946,55.499079 79.500097,55.499079 z">
<Path.OpacityMask>
<LinearGradientBrush StartPoint="0,0" EndPoint="1,0">
<GradientStop Color="Transparent" Offset="0.3"/>
<GradientStop Color="White" Offset="0.3"/>
</LinearGradientBrush>
</Path.OpacityMask>
</Path>
<Path
Fill="Transparent" Stretch="Fill" Stroke="Red" StrokeThickness="1" Margin="-0.5,-0.5,-0.5,-0.019" Data="M63.50031,-0.50054431 L47.500523,55.499079 -0.49883747,55.499079 39.50063,95.498578 23.500843,159.49799 63.833676,128.4989 103.7623,160.51698 87.095797,95.850405 127.49946,55.499079 79.500097,55.499079 z"/>
</Grid>
</Page>