There is a traditional art form called Urushi lacquer in Japan.
There also is a plastering technique called Suna-Kabe (sand wall) this is a mix of sand and glue.
The bathtub was created with these methods.
The red is all the Urushi lacquer finish. The black is natural iron sand, and the blue is crushed jewels called Lapis Lazuli for the Suna-kabe finishes.