Perhaps nym being a maritime nation has something to do with it? Otherwise you may as well ask about the rationale behind half the other soul crystal colour schemes. What colour would you have chosen? Brown is generally avoided as an attention grabbing colour so that’s out.

Frankly a lot of the new jobs don’t have coherent identities, particularly when compared with the base game ones. Summoner and healers being gutted has done a huge amount of damage to the identity, but it’s still clearer than gunbreaker or sage.