: ‘Cannot animate ‘Fill.Color’ on an immutable object instance.’

Create a new SolidColorBrush as value for the Fill Setter:

<Style TargetType="Rectangle">
    <Setter Property="Fill">
        <Setter.Value>
            <SolidColorBrush Color="{Binding AlertUnit.AlertColor.Color}"/>
        </Setter.Value>
    </Setter>
    ...
</Style>