The blue color he mainly used is called “Hokusai Blue”, which you can find in “The Great Wave Off Kanagawa”, one of the series “Thirty-six Views of Mt.Fuji”. The color was not originated in Japan but imported from Berlin, Germany. At that time Japanese called it Bero-Ai means the blue came from Berlin. Bero-Ai’s pigment was the same with Prussian blue and new color to Japan at that time. Promoters of the color intended to spread it by Hokusai’s and any other artists’ works. That was the marketing in the Edo era. Please refer to “Indigo Blue”. You will find the history of Japanese blue color including Ai.