I guess it's gonna be just like in Cyberpunk 2077 where it was so diverse that the only blond hair, blue eyes, white male in Night City was the character you created...
I haven't really been paying that much attention to most of the media about this game prior to release other than a few trailers (trying to avoid spoilers), so I am not fully up on whatever the starfield lore is.
While I dislike when fantasy or historical settings randomly insert ethnic diversity without a good explanation for it, most people being brown is probably a pretty accurate view of a future space society. With increased ease of travel and communication the human race will continue to homogenize ethnically, and white is definitely a global minority even as of right now.
Even significant colonization within our solar system is decades away at the earliest, while interstellar colonization is much further out than that. So we will be even further ethnically homogenized by then, as well as having increased ethnic diversity in all areas from the easier movement of people globally.
Birth rates also trend that humanity in general will be even browner in the future with most of the significant population growth we are seeing and expecting to see being in Africa.
The only way to support significantly different ethnic make ups in the far future would be if somehow some colonization effort deliberately made the effort to be from one specific ethnic group.
The only ethnic group I could see possibly achieving that would be the Chinese. Due to China's potential to manage a Chinese only colonization effort (although given the likely size of such an effort it may not be possible for any one nation to do it themselves) and because China is still relatively less diverse with a government that may enforce much lower rates of accepting migrants (although their demographics are about to fall off a cliff, so maybe importing labor with gusto will happen soonish for them).
Another realistic scenario could be that the more developed parts of the world take the lead, and provide most of the people, for interstellar colonization, while global ethnic homogenization is still not complete. But that wouldn't mean white. That would mean that earlier colonization waves would be lighter brown (higher original input of east Asians and white) compared to later colonization waves which would be darker.
The other potential source of ethnic differences in the future is of course genetic engineering to adapt to different environments. And maybe that might include engineering lighter skin as part of that.
But outside of that I don't see any reason to expect a significant number of white people in a future sci-fi game that takes right now as the starting point. The only things that make sense realistically are Chinese (maybe), light brown, and dark brown.
The actual issue with realism is that of most sci-fi portraying the moderately distance future being unrealistically white.
Based on these complaints, Starfield seems to be taking their world building seriously and taking a realistic view of things.
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