To account for ethnic diversity on the planet, people who hold to a 'literal' view of The Flood and also ascribe to a Young Earth Creationist (YEC) view (Earth is several thousand years old) have to move the creation of the diverse genotypes which underlies the diverse phenotypes that make up contemporary humanity into a very compressed period.
This site, which has a YEC point of view, claims "The Biblical data places the Flood at 2304 BC ± 11 years." That would be 2304 + 2021 = 4324 years ago for the genotypic diversity to occur.
Yet, it would have had to happen even quicker than that, as we know there was significant genotypic diversity well before the present day, due to various historical accounts. So we're probably talking at most a couple thousand years to create at least a significant part of the genotypic diversity we see in the world.
At 20 years per generation, that only allows 100 generations for specific genotypes to emerge from the genetic bottle-neck of the Flood. (Note that it is not clear how long various generations would be, given the age of humans is longer at the time of Noah per the Bible, and so there might be significantly fewer generations than this.)
This answer is perhaps the typical answer to the question: "The physical characteristics of various ethnic groups developed in these isolated groups due to the sorting and isolation of genetic information already present in man’s DNA. The information in that DNA had originally been present in Adam’s DNA and has just been shuffled and sorted. Therefore, the development of so-called races has nothing to do with molecules-to-man evolution. All people groups are still humans, descended from one original human."
In other words, the development of ethnic differences would be similar to the process used in the development of dog breeds, where the emphasis isn't on novel genetic mutations (say) but selective breeding based on an existing genetic repertoire. Of course, humans exert extremely strong selective pressures to create new dog breeds, and it's not clear how analogous natural selective pressures would be in this regard.