I'm not talking about whatever your specific use case is, I'm only talking about how NodeBB/database works.
If you don't see MongoDB suitable, because its collection model is not used by Nodebb, the table model around PostgreSQL wouldn't be used either with the additional problem of being tied to an specific schema if you don't model all data with key/blob.
What I'm saying is that the relational table structure (SQL) is closer to key/value (Redis) than the document-store structure (Mongo). Obviously that doesn't matter when talking about NodeBB/database since it doesn't using any of those specific features. It essentially just emulates Redis on top of Mongo, which a Postgre adapter would also do.
@Ryan-Feng I believe this can be done with a change in the theme templates for your site. We do send over the group label, and that should contain the colour hex value, which you can use as a style attribute for the username itself.