Я даю вам простой пример, в котором я показываю две кнопки и одну сетку, вы можете запускать ее с любым разрешением, левая и правая кнопки всегда остаются в одном и том же положении, и сетка будет меняться соответственно размеру разрешения
<Grid HorizontalAlignment="Stretch" Margin="0,0,0,0" VerticalAlignment="Stretch" >
<Grid.RowDefinitions>
<RowDefinition Height="150"/>
<RowDefinition Height="169*"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="150"/>
<ColumnDefinition Width="113*"/>
<ColumnDefinition Width="150"/>
</Grid.ColumnDefinitions>
<Button Content="Button" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="10,55,10,40" />
<Button Content="Button" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="10,55,10,40" Grid.Column="2" />
<Grid Grid.Column="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" BorderBrush="Black" BorderThickness="2" Grid.RowSpan="2"/>
</Gid>
ваш пример кода
<Grid HorizontalAlignment="Stretch" Margin="0,0,0,0" VerticalAlignment="Stretch" >
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="945"/>
<ColumnDefinition Width="945"/>
</Grid.ColumnDefinitions>
<TextBlock Text="Color"
Margin="117, 84, 0, 0"
FontSize="48"/>
<Button Grid.Row="1" Content="Red" Width="500" Height="80" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
Margin="445 30 0 0"
/>
<Button Grid.Row="1" Grid.Column="1" Content="Yellow" Width="500" Height="80"
Margin="0 30 445 0"
HorizontalAlignment="Center" VerticalAlignment="Center"/>
<Grid Grid.Row="2" Grid.ColumnSpan="2"
Margin="84 40 84 0" HorizontalAlignment="Stretch"
VerticalAlignment="Stretch" BorderBrush="Black" BorderThickness="2"/>
</Grid>