Recently read John Scalzi's (at Whatever) explanation of why he rarely gives his characters a skin color and Kameron Hurley's (at Brutal Women) response to Scalzi's post. Here's my thoughts on it.
As I read it: Scalzi is saying he doesn't give his characters a skin color because it isn't important to the story. If it is important than he makes sure it is in there. He does, however, write non-white characters; he just doesn't go around describing their skin color. Hurley is saying that if a character is explicitly not white, middle-class male than that is the default. Hurley also says that it is important to make the distinction because there are cultural differences.
First off, in three of Scalzi's books the main characters are GREEN. The bodies grown for the CDF soldiers are green to make them easy to identify. It also means that based on skin color there is no indication of country of origin or ancestry, so one has to resort to guessing by name. Presumably, when their tour of duty is over new bodies the soldiers get have whatever skin color originally encoded for in their genes.
Scalzi doesn't ignore race in the Old Man's War universe. Soldiers come from developed nations (USA, Canada, western Europe, China,...) and colonist come from third world countries (India, Sub-Saharan Africa, Southeast Asia...), IIRC. Apparently, the racial politics are even directly part of The Last Colony. As I remember characters have varied names suggesting different countries of origin and/or ancestry but there is no big deal made about it.
Science fiction stories usually take place in future; only a few are set within any of our potential lifetimes and many take place in the distant future. That means that the cultural differences that exist today most likely do not exist at the time of the story. Even a hundred years could be enough to erase the differences in east coast inner-city boy and mid-west farm girl. Some differences will remain whether within a country or around the world, but many of the differences that are seen in today's society could disappear; particularly those primarily due to differences in ancestry because we are becoming a more homogenized culture (for better or worse). I like to think that the lack of importance of race in many sci-fi books is the result of the author's desire to see race be less important; they really want the world to be color-blind, at least within one a give country. This is probably a projection of my desire for there to be less emphasis put on one's skin color.
Another example of race/culture in literature is in the Harry Potter books. J.K. Rowling has students of many different ancestry at Hogwarts. Harry, Hermione, and Ron are Anglo-Saxon, Blaise Zabini is African, the Patil sisters' names suggest they are Indian, and Cho Chang is a Chinese name; and they are all English. I'm sure there are some Welsh students. Professor McGonagall is Scottish. Seamus is Irish. Most students are "white" but the school is in England. What do you expect? They reflect the population of Great Britain.
Ms. Hurley, my primary problem with your rant is, is the difference between an inner-city Hispanic-American boy and an inner-city African-American boy more or less than the difference between an inner-city African-American boy and an upper class African-American boy?
I don't think so. Certainly using characters from different cultures opens up different potential sub-plots, but culture is not based on skin color. It is based on where one was raised.
Slight aside: Scalzi's successful creation of a character without specifying gender ( scroll down to April 18th, for his explanation) may suggest that Hurley is wrong from the get go since being named 'Sam' and in a relationship with the male, Archie, did not make Sam a female for all readers. Most readers would have imagined Sam as male or female but some may have imagined intersex. I haven't read Android's Dream, so I don't have an opinion, yet.