If it is not working, then you need to point to the property using
Path
:
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Foreground="{Binding ElementName=Rectangle, Path=Fill}"
Text="MyText" />
<Rectangle Name="Rectangle"
Grid.Row="1"
Fill="Red" />
</Grid>