Even Korean descendants who were born and raised in Japan prefer to keep their nationality as "Korean", which is rather odd, but understandable. As for Japanese initial colonization, I haven't heard of this version when Korea would had been colonized by people coming from the Japanese islands. The history I've heard tells the contrary - the people from the continent (where now is Russia) came to Hokkaido (Japan's northern island) during pre-history, which is about 40,000 years ago, and settled down to become the Ainu. They were the first inhabitants, but looked nothing like the Japanese look today. Some of them still don't nowadays, which is odd even to the Japanese. They remained in the north, though. It was much later that the main island (Honshu) was inhabited, and there is a lot of controversy about who they were, but it's certain that they came from the continent - probably Korea and/or China. It is more or less accepted that the Japanese have a common ancestor with the Mongol people. No wonder the main island was uninhabited for so long - 70% of it is covered by active volcanos and high mountains, making only 30% of the land inhabitable. Even nowadays, we have about 50-80 earthquakes EVERYDAY in Tokyo. I don't miss that part. One funny thing is that the origin of the Japanese language is mostly unknown. It has features in common with German, which makes it even more mysterious.