How to get array of unique array values across rows?

I have a field containing a set of codes in a varchar array, each tied to a person.

I'm trying to query this info so that I can get the list (presumably in an array) of all the values in this array, across all the records for a client. So that if a person has two records, one with ORANGE and BLUE, and one with BLUE and GREEN, I could end up with a list of ORANGE, BLUE and GREEN.  

I had hopes for:

SELECT client_id,array_agg(service_codes) FROM foo GROUP BY client_id;

But was rebuffed with "ERROR:  could not find array type for data type character varying[]"

Ken
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