This article shows how to synchronize a label custom field.
Assume you want to synchronize the label custom field 'label - cust1'
Set the received label to the local label custom field and if it doesn't find one, it sets an empty array so you don't get a null pointer exception.
This method overrides all the existing labels in the custom field.
Add the received label into the local label custom field
How to set a default value in a label custom field
To set a default value in a label custom field use the nodeHelper.