The last day of the year it is almost mandatory to enjoy soba, a symbol of long life and thin healthy lifestyle. The types of flours used range in color from brown, white to green made with tea powder (pictured below) is yomogi soba are soba noodles made from buckwheat and wheat flour, mugwort powder, and water.
Other colored-soba such as black uses sesame, yellow colored soba is made from egg, and pinkish colored soba is sometimes made using ume, Japanese plum. Depending on the shop, the percentage of buckwheat flour in soba noodles typically ranges between 40% and 100%.
The seed of buckwheat is layered and each of the layers has a different color and characteristics. The outermost part is the chaff, colored dark-brown and rich in fiber. The layer which is just inside of the chaff is called endocarp during the buckwheat season is a pale-green and is the most aromatic part. The next inner layer is endosperm, it is white, and contains plenty of starch. The core is the embryo bud, pale-cream and like the endocarp it is rich in protein.
The first flour from the mill, (endosperms) is colored white with its main ingredient is starch. After the first flour, second flour is collected from the mill. This flour is a mixture of the remaining endosperms and the embryo buds. Because of the embryo buds, the second flour is pale greenish-yellow or sometimes brown. The texture of soba made from the second flour is said to be smooth because it contains less fiber.
The third flour is made from the all of the remaining parts, which are gathered and milled again. This flour consists of the remaining endosperms, embryo buds and endocarps. Its color is pale green or dark-brown.The aroma is stronger than any other types of flour, sticky and rich in nutrition. However, because of the higher quantity of fibers from the endocarps the texture is inferior to the first and second flour.
Whole grain flour is the flour that results when all layers of the buckwheat seeds, including the chaff, are milled together. This flour has the all characteristics which the first, second, and third flour have. It has a blackish tinge which is derived from the color of the chaff and the texture of the soba coarser.
source: internet, etc.