Japanese ships of WWII came out of four main shipyards. There colors, topside, depended upon what shipyard they were assembled in. The yards in question were Kure, Maizuru, Kobe, and Yokosuka. Model Expo in Florida has a color guide of Japanes ships during that period. They also sell a line of acrylic paints which match the color chips for each of the yards.
I-19 was built by Mitsubshi at Kobe and therefore that color grey would have been used for that boat. The few colored pictures I have of Japanese subs will show a rust color below the water line. I would assume that could be some form of anti-fouling red. I won't swear to it. Hope this helps.