Because the Setter
element doesn't support direct content, you must specify that you are setting the Value
property (include "<Setter.Value>" in your XAML):
<Setter Property="Background" >
<Setter.Value>
<MultiBinding Converter="{StaticResource dataToColorConverter}" >
<Binding Path="Color" />
<Binding ElementName="myListBox" Path="SelectedItem" />
</MultiBinding>
</Setter.Value>
</Setter>