Honestly? I think a writer should briefly mention the skin-color or texture (because maybe they’re humanoid, but not actual humans, so they may have dry or slimey scales, etc.) because it adds more to the story. And maybe there aren’t just white (Caucasian) or black/dark skinned people either. Maybe your humans have grey skin or they’re green or maybe they’re peach-colored?
I understand you don’t want to add someone’s skin type because of racial issues and the fact that it may not have anything to do with the story, but that’s from afar. Close up, it actually has a meaning: allowing different people to be represented. Some authors don’t add it at all into their story, and that’s fine because some can get away with it, but this can effect your readers in a negative light because many will see that you don’t say it and then automatically assume everyone is white. It happens more often than you think. It even happened to Hermione Granger (from Harry Potter). Her skin color was never described, so many assumed she was white. They even casted a white actress to play the part. It wasn’t until the stage play where a black actress became Hermione when everyone went up in rage: “She’s white!”
This can also cause problems with POC readers because they might think they’re not getting represented either, and may even pass up on the book just because it may feel as though there are no POC characters or that it’s “another white cast” if they’re looking for books with diversity.
Skin color isn’t always everything, but it doesn’t take very long nor is it hard to briefly mention it in a sentence that they have dark, olive, tanned, taupe, etc. skin. And if your characters are not from Earth, then they are of a specific race because there is no such thing as an African-American or an Egyptian or Polynesian from a world unlike ours. They’d have to be another race (a name you create), and therefore, they’ll have to have their own culture and lifestyle that you also create. You can base them off real life people, but still make it fictional and still make it to where it’s not focused on, but at least briefly mentioned so you A) represent various types of people, and B) give more depth to your world and story.
Take Avatar: the Last Airbender as an example. Completely fictional world with its own races, cultures, etc. although the world is focused/based off of Asian and Native American races and cultures as the Water tribes are based on Inuit culture, Earth Kingdom is Chinese culture, Fire Nation is Japanese culture, and the Air Nomads are based off of Tibetan culture. Their races nor their cultures are specifically stated because they are fictional, but you see it through how these characters interact in the world and visually seeing various things like architecture or clothing that tells you, “Oh, this is based off of this culture.” But Chinese, Japanese, etc. races aren’t said as they don’t exactly exist as we know it in the story. These characters just… exist. And that’s that.
That also brings me to the comment about “to write humans existing” because I’d like to add something: this kind of comment can be said about many other traits. Deformities, learning disabilities, body types, sexualities, so on and so forth. We all exist, and there are humans like this who exist on other planets (or could, hypothetically speaking). But we all want to be represented in a way where we can see ourselves and be happy that someone else saw us for who we are. If you replace “race” with any of these other options, like deformities or body types, then you’re erasing that representation. It’s fine to want to escape in a world where there are no real world problems, but when you say you just want “humans to exist” you have to bring those real world problems into the equation because simply existing doesn’t exactly cut it when nearly every book out there doesn’t have an overweight character or a character with no arms or a character who is transgender or a character who is deaf or a character who has schizophrenia or a character who has so on and so forth. The readers who want to see diversity don’t get to see it because all they see are people, and it’s okay to see people, but people exist in all shapes, sizes, and colors.
It’s just like white room syndrome. This is a common mistake for writers who can see the world in their head, but don’t write it down, and the readers have no idea what the world looks like. It’s a blank canvas to them. If you don’t give them those details you have in your head, they can’t see the paint. It’s invisible to them. You have to draw it out yourself to make sure they get the picture.