Just to clarify - I don't put in descriptions because most of the time, they're not important to the story. So, if I toss in a PoC for the sake of diversity, I'm messing up - but if I don't include PoC then it's just 'business as usual' and that sounds like I'm messing up. But I guarantee, if I put in a character based on some of the PoC I know personally, I'd get in trouble for that, too - either because they were too "white" or because they were "stereotypes". It all depends on who's reading it.
So, yeah, business as usual or not, I'm not putting anything in that isn't important to the story.
The default is white male. I tested this once.
So I wrote a short bit with a guy talking about this magical object and the female reader assumed right off from the talk that (though there were no pronouns referring to the characters) the characters were both male.
Default is therefore white male. (Christian)
Now you're trying to say, "Well that's just downright stupid." It is. That's the point. The whole culture is stupid to think this and default it this way. But it just BE that way. The culture programmed people to think in this fashion, so by outing POCs and whites equally in the story as being so, you could fight it.
Case studies: Last Airbender on racebending. The leading character on the cartoon is "Aang." It was made into a movie. The director is Indian. The name Aang is clearly Sanskrit. The kid is a poster child for being a Buddhist Monk. Who did they cast? A white kid. (I kind of want to take the director and yell at him, you're putting down your own culture you bastard!!)
Case study: They made Hunger Games character of color like in the book. Despite the book repeatedly stating the character was of color white people were outraged, OMG, how can you make her black. You mean the part where the book said she was black? They'd defaulted the character to white.
Is it stupid? Yes. Is it wrong? yes. But this is what you're facing. Systemic racism.
I put in descriptions in the stories because it gives a picture. Like a setting gets a description. A character I give a rough description. Also the name often defaults characters (though some readers are just that stubborn and want "Akira" to be not Japanese. (See Racebending on the movie project).
The question is fair representation of the diversity in your population. Humans have been traveling around back before they were considered human (See Homo Erectus. They spread throughout Asia. Also summarized as the "Out of Africa" v. "Multiregional hypothesis" problem). Which means that populations without fancy things like planes and huge boats have been traveling great distances--there have been populations moving, the Romani were originally Indian (Indo-European which means you have three migrations in there--Africa, Europe to Asia, and Asia to Europe).
So the argument that Europeans in Medieval times would never have heard of anyone of color is kinda dumb. You had people of color in Rome, but it wasn't made a big deal of as class was. There are large bits of variety in populations, such as white people in Asia (There is this HUGE country in Asia called Russia... also the Tocharians, Ainu, etc.)
So when people ask for diversity, they are asking for fair representation of human populations in order to be realistic.
Now, about including PoCs and doing it right... you have to ask if your society cares about it and what stereotypes you are holding onto and be willing to adapt and apologize at the drop of a hat. Also eat lots and lots of humble pie. But being at the top rung, it ain't so easy to swallow humble pie. I understand there is a great sense of resentment at doing so and just wanting to give it all up. It makes you wonder why you are writing PoCs or in a country/culture not your own. What deep conviction and so on. That's good.
Let's be straight up. I'm Korean Adopted to a Jewish family writing about Kushana people (India for the history-challenged), Koreans, Japanese and Chinese. In it I also mention Greek, Romans, Bactrians and Persians (I think it's Persians.) As well as some other Asian groups... It's a pain in my backside to have so many groups. I don't expect the reader to keep track of it, but I still want to represent all groups mentioned fairly within the context of the story even if the research will be longer. It gives the reader a fairer representation of the period.
Still, I have a bunch of cultures I do not know on my hands and I'm super nervous. When I started, I checked with my friend from India and promised her the following:
1. I would not use her as the sole source of how to be Indian.
2. I would be humble and please smack me if I get out of line or ask too many questions. I repeated this a few times.
3. I would check with other people who were Indian as well.
4. If I get annoying, she can tell me to stop.
5. Let a group of Indians read the results.
I also checked against Chinese and Japanese, looked up stereotypes within and from outside the countries, checked on the literature, movies, and media representation within their country and within our own country. I read everything I could historically and sorted out the information. Since I'm an insider and outsider to Korea, I also triple checked my stuff against that. Still after all of that I'm willing to say I ef-ed up. To apologize and say, "Yup, I really screwed that up."
I rather write about a diverse group of people, f it up and eat humble pie and say sorry, I won't do it again that stick to safe and write about only white Christian people. My reason is that it gets tiring to write about the same group over and over again when the world is so rich with a variety of peoples cultures, individuals, fauna, flora temperatures and climates. I think the diversity is cool in the broader and smaller strokes. I want to write about HUMAN experience. Not just white Christian experience. So in order to do that, I am very willing to eat humble pie. I'm willing to get it wrong and be slapped for it. I won't make excuses and say "Well some PoC should have corrected me." I'll fight stereotypes and examine them in order to do that. I'll write women as leads, people with disabilities, people who are of color, people who are QUILTBAG, different religions, people with rich family backgrounds and so on in order to do that. Because I don't want to limit the kind of story I can write, and I don't want to live a lifetime of having it wrong in my head either.
From writing about India and willing to get it wrong, I learned a whole, whole lot. The picture of India in my head has been corrected by a lot of degrees. I'm grateful for that. Because learning one more culture and the diversity in it means I can see humanity better and the act of writing is often the act of understanding what makes us human. (I should note I scrapped it at least three or four times from getting it wrong. Even my friend didn't know some of the stuff I dug up...)
If someone tells me if and when my book gets published that I'm all wrong, I'll eat my humble pie and swear to do a better and fairer representation next time and try that much harder. That's how it goes.