The background which was dyed by the Rou-tataki(spraying of wax) which is one technique in batik dyeing. That produces the complicated colors like Pointillism.
To dye a background, a design part is stopped by wax. Gojiru※ is pulled by a brush as it won't be a dying blur. After that is the 1st time of background dyeing (pink). By rhythmically hitting the brush soaked in wax, the wax is scattered finely. The other colors are dyed while repeating this work.
※ Gojiru：Liquid made from the protein of soybeans. The dissociation of the dye and the water is stopped.