Set foreground back to default color using csharp

            <TextBox Header="Name"
                     MinWidth="250"
                     Grid.ColumnSpan="2"
                     PlaceholderText="Your pet name"
                     Text="{x:Bind Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                     Foreground="{x:Bind NameErrorsList.Count, 
                                         Mode=OneWay, 
                                         Converter={StaticResource ErrorListToForegroundConverter}}"/>

    public class ErrorListToForegroundConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, string language)
        {
            var errorCount = (int)value;
            return errorCount != 0 ? new SolidColorBrush(Colors.Yellow) : null;
        }
        public object ConvertBack(object value, Type targetType, object parameter, string language)
        {
            throw new NotImplementedException();
        }
    }

return errorCount != 0 ? new SolidColorBrush(Colors.Yellow) : null