Here's an attempt at enriching the borderpool in China. Added borders correspond to the Songhua, Yellow, Huai, and Yangtze Rivers (though the outdated borders made it difficult to draw west of Wuhan), Qinling, Hulunbuir, and the area where Koreans are concentrated in China.
Added colours:
Manchuria, for Heilongjiang east of the Songhua. Rationale: This area, including Harbin, used to be part of Jilin instead of Heilongjiang.
Yanbian, for Koreans in China. Rationale: Yanbian is the autonomous prefecture for Koreans.
Nanjing, for Jiangsu and Anhui south of the Yangtze. Rationale: While Nanjing is the provincial capital of Jiangsu, most of Jiangsu still lies within the maroon area (Jiangsu colour).
For the south, I may go with the Pearl River, ethnolinguistic groups, and the Pearl River Delta region.
