I'm your friendly local brown person who is visibly different from the majority of people in the country.
I'd like to try and explain why having some outwardly, non-choosable feature of my physical self being part of a costume feels really, really rancid.
It's because when you dress up as someone, you don a feature that makes them who they are, based on how you see them. If you dress as David Bowie as a tribute, you'd wear a blonde wig and his outfit from Labyrinth, yeah?
Let's say you decided you wanted to dress as Ketan Joshi, and you don some glasses, a fake beard, a red skivvy and a tan blazer. Cool! That's what I look like.
Except, you're not finished. You coat every visible inch of your visible skin in thick brown paint, and "now I'm done. Now, I look like what Ketan looks like, to me", you say.
This is a signal that the wavelength of light that my outer layer absorbs is a major part of what I am to you. To the point that you feel I'm unrecognisable without being designated as A Brown Man. You see me as an amorphous, brown figure -- a part of me that I had no involvement in creating is, according to you, a major part of me. I can never change that. That is how you see me: a brown man.
This is why it really hurts people who belong to a group that is visibly different, when you collectively rise up and say to them 'a feature of your body is a costume, to me. It's a bit of you that you can never disown, and you have no say in how much of you it comprises. We decide, and you have no power to stop this. Even if you ask us to stop, we won't. If you tell us it hurts, that it cuts you to the core, we won't stop'.
So, you know. Consider that it hurts, not because of some evil push for political correctness, but because it actually hurts -- because we're not lying, or faking it. Because you actually cause harm, and maybe if you don't get that much joy from doing it in the first place, consider simply not doing it, or calling out others when they embark on the process of hurting people.