Property For MVVM Trigger

Test.xaml

    <Label Text="{Binding LocationLine}" FontSize="Small">
                                <Label.Triggers >
                                    <DataTrigger TargetType="Label" Binding="{Binding LocationLine}" Value="NA">
                                        <Setter Property="TextColor" Value="Gray"/>
                                    </DataTrigger>
                                </Label.Triggers>
                            </Label>
                            <Label Text="{Binding ContainerLine}" FontSize="Small">
                                <Label.Triggers>
                                    <DataTrigger TargetType="Label" Binding="{Binding ContainerLine}" Value="NA">
                                        <Setter Property="TextColor" Value="Gray"/>
                                    </DataTrigger>
                                </Label.Triggers>
                            </Label>

TestViewModel

 public string LocationLine
        {
            get
            {
                StringBuilder sb = new StringBuilder();
                sb.Append(this.Location);
                if (this.StartAirport != null || this.EndAirport != null)
                {
                    sb.AppendFormat(": {0} - {1}", this.StartAirport?.icao, this.EndAirport?.icao);
                }
                if (sb.Length == 0) sb.Append("NA");
                return sb.ToString();
            }

        }