It's more like coloured ink wash, it's like a thicker paint than watercolour for sure. How Stan coloured a LOT of his pieces that he did in the 80s. He stopped doing it (along with stamping with the red hanko mark) around 1992, although I do own a piece from 1996 that was coloured by him, probably due to time constraints, as he started to get ever more popular.
https://en.wikipedia.org/wiki/Ink_wash_painting