A recent Final Fantasy XIV video looks at the creation of monsters in Shadowbringers.
The video looked at the concept, creation, and design of these monsters. One monster was the Sin Eater. The team described deliberately using white as the color because this color is relatively unnatural, creating this juxtaposition.
Another interesting factoid was the sheer time it takes to develop a boss monster. The team stated that this can take up to six months from the concept phase to the end phase where the monster is effectively completed.
Be sure to watch the whole video below for a more in-depth look at how monsters are developed for FFXIV.