Hi. I would love to see a more diverse set of portraits in Sulis. I definitely would not oppose any PRs adding additional portraits, as long as the work of properly attributing those is done.

However, just on a personal taste level I dislike the style of the Flare portraits. I could be wrong, but from my cursory glance it looks like many (most?) are actual photographs of people with a watercolor effect applied. There is something to be said for any process that allows you to include substantial art assets without major effort - but in this case, I don't think it is for me.

It would be fairly straightforward to add a mod adding additional portraits. You would need to add the actual art files, and the corresponding YML spritesheet definition, lets say it is called flare_portraits. Then, add additional keys under portrait_selections in the appropriate file in your mod directory, i.e.:

races/human.yml:

id: human
portrait_selections
  - flare_portraits/human01
  - flare_portraits/human02

This would then get merged into the base human.yml file when the mod is active.

Some ethnic portraits

Pretty sure some can be found in wesnoth's huge data set.
On my machine I could only quickly find on dunefolk portrait (herbalist) but I'm still running 1.14. It's very likely there are more of those in more recent version (stable is 1.16 those days), which are likely from mongolian inspiration in the drawings.
Pretty sure I've seen portraits of people with black skin too in the many mods I've played...

Note: I do not really mind, it's just to give a pointer for someone who would want this enough.

Something worth checking out https://www.fantasy-faces.com

Mostly white/Caucasian but ethnic portraits also happen from time to time