{RelativeSource Self}can't work in this particular case as your path changes it's value dynamically ;-)
<ComboBox Width="Auto" Height="Auto" IsSynchronizedWithCurrentItem="True" x:Name="ComboBox" IsDropDownOpen="False" SelectedIndex="0"> <ComboBox.Foreground> <Binding Path="SelectedItem.Foreground"> <Binding.RelativeSource> <RelativeSource Mode="Self"/> </Binding.RelativeSource> </Binding> </ComboBox.Foreground> <ComboBoxItem Content="Apples" Foreground="Red" /> <ComboBoxItem Content="Oranges" Foreground="Orange"/> <ComboBoxItem Content="Pears" Foreground="Green"/> </ComboBox>'Longhand' version but it could be stated as an attribute of ComboBox. But admittedly it's a far more convoluted solution than yours <s>