When your shopping list arrives the notification will light the LED in your preset color. The problem is that your "new email" notification comes a split second after and overwrites YGS+ notification.
Go to Gmail web interface and create a filter that will mark all messages with your predefined YGS+ subject as "read". This way you will not get "new email" notification for this message on your phone.
Do not make the filter to archive them! If you archive them they'll not come to your Inbox and will not be picked up by YGS+.