To what extent is it appropriate to add fandoms that have just a single chromatic character to this project, especially if the rest of canon is mainly white?

One of my yuletide requests is Susan Cooper's Seaward which is, among other things, an interracial romance and Westerly is awesome so might qualify. On the otherhand it's a story where teenagers from our world fall into another world and the created world is largely Celtic in its mythology. I also can't recall anything that would place West's specific ethnicity, nationality, culture or region of origin although he's clearly neither white nor European/North American. Hence issues. But on the third hand, I would dearly love to see West's backstory and identity fleshed out in a way that made sense in real-world history. (He does have some detailed backstory including a disappeared father and a possible coup going on when his story begins.)

I don't want to clutter up the community with inappropriate fandoms; on the otherhand a quick internet search makes me feel I need to stand up and shout "West isn't white. Please don't write him as if he is."