From the beginning, the Cluster has been explicitly multiracial and multicultural. The vast majority of slow boat settlers were from UN ships, with settlers from a purposely varied group of nations and peoples. There are very few which came from single nations, which are limited to the ethnic mix of that one nation.
Why? For a lot of very good reasons. One, the UN is made up of many different nations, and by assigning passengers and crew from multiple nations to each ship, it could guarantee better representation from each nation and people. Two, people from very different ethnic mixes would give more genetic material to choose from, a far better evolutionary situation than a monoculture, so that the colony would have the best genetic chance of success. Three, by sending a mix of cultures, it could better secure funding to make the ships in the first place. So, Politics, genetics, and money all entered into the decision-making process.
To prevent ethnic cliques and cultural dominance games, with consequent cultural breakdowns over the long (1200+) years of the sub-light voyage, most of the ships used cultural emulation, where all of the people in a ship or, more commonly, in a watch - that is, all the people who were awake as crew at the same time - were encouraged to adopt an emulated culture and language different from their own. Usually, over time the synthetic culture became the natural culture, without reference to actual ethnicity. This procedure was not forced, but was voluntarily adopted on most slowboats, because of a greater chance of coherence over time.
So, the people of the Cluster were ethnically and culturally mixed long before they got to their destinations. The genetic make up of each world is different due to the different origins of the initial crew, and thus the world had it's own particular ethnicity - most tending to brown skin, dark hair, and brown eyes with a slight epicanthic fold, but all unique, all varying, and none quite like the ethnicities of an Earth long gone.