The Desert Mermaid

I worry about how to handle this a lot... in my head, my books have a lot of genetic and racial diversity, but I don't really picture any PARTICULAR character as having ANY particular skin tone, light OR dark OR a unique variation (I really just don't have very clear pictures of anyone). So my general policy is that if I mention ANYONE'S skin tone, I have to try to find a way to mention EVERYONE'S, so that if I do mention that someone has a darker shade of skin, the readers won't go around assuming that that character is the ONLY darker-toned person in the book. On the other hand, more often than not I really don't CARE what skin tone any of the characters are, so I don't feel a compelling need to address the issue AT ALL--but if I mention no one's, I worry that people will assume that everyone is white, when really, you're welcome to picture them as whatever you want, because they just as easily CAN be whatever you want, and there is no "canon" about the issue at all. Finding a way to mention EVERYONE'S is tedious, but I don't want it to be seen as White As Default, and ergo they're white unless otherwise specified. On the OTHER hand, I don't REALLY want to lock all of my characters to ANY shade, because that comes with a whole other set of potential tricky points (e.g. I don't want people to think I'm just tossing in Token PoC, even if I make the ratio balanced and non-biased amoung the range and roles of characters; and also, if a person WANTS to picture everyone as black, for instance, then who am I to tell them that they're NOT?). This issue drives me crazy, because I really don't know what the best approach is.