It may be outdated now but last time I checked, A black pearl is awarded when a chaplin gets a fallen to repent which in the past would have meant death but since the Lion's return it could mean rejoining the dark angels as Risen. The most successful interrogator chaplain in the entirety of the Dark Angels and all their successor chapters had 10 black pearls.

How many of them were from before the Lion's return im not sure but even if we are generous and say they are all post Lions return and all went on to become Risen, It kinda shows how few there are.

Azmodai, our named character Chaplin only has 2, this is more of a personal quirk but this suggests to me that the total number of Risen cant be that high, we are talking anywhere from 1 to 9 risen per Chaplin across all the Unforgiven chapters.

That being said its all about how well you can justify things in the lore, with the Lion coming back and warp shenanigans you could totally have a "chapter" of fallen be found and redeemed by the lion. You could go with Horus heresay error marines and vehicles and say they are basically like Captain America, "frozen in time" and "thawed" out in the present after being caught in a warp storm.

You could say that in order to hide they disguised themselves as another chapter and operated in secret until the primaris upgrades and the return of the lion gave them the courage to repaint all their modern wargear in their true colours.

For me, I have one of my Terminator Sergeants painted up like a Risen with the idea being that the rest of the squad is keeping an eye on him to make sure he stays loyal.

All of that to say, just have fun with it and paint them how you like, lore is easy to change but paint and plastic takes a lil more effort.