A paint brush or ink brush was a tool used to bring up paint on a model or painting.
In 2121 a young Jonathan Archer used a paint brush to color his remote controlled spaceship. (ENT: "Broken Bow")
Keiko O'Brien's grandmother, whom she called obachan, used an ink brush when creating pictures of Japanese calligraphy. (TNG: "Violations")
Captain Jean-Luc Picard also used a paint brush while painting. (TNG: "A Matter of Perspective")