Hospital ships are not camouflaged. According to international conventions they are supposed to be distinctively painted, exhibit lights at night, and maintain a steady course. These conditions are expected to give them immunity from attack. Notification of their existence is to be made via diplomatic channels.The following sketches show the appearance of Japanese hospital ships during WWII. One of them, Hikawa Maru, survives to this day as a museum ship in Tokyo
One Japanese hospital ship has a unique story. The ship was originally the Dutch hospital ship, Op Ten Noort. She should have been immune from capture because of her status. But the Japanese took her over, and by degrees, put her into their own service as Tenno Maru. Realizing that they were in violation, they reported the ship had struck a mine and been sunk. At the same time, they put her into a shipyard to change her external appearance, and renamed her Hikawa Maru 2. At the end of the war she was scuttled. She was discovered several years later, identified, and the story became known. Japan ultimately paid the Netherlands compensation for her.
Wetherhorn 