An actor's onstage assistant is called a Koken. Koken appears on stage in various costumes including montsuki hakama and kamishimo (formal samurai clothing) depending on the scene and the work. A Koken whose face is hidden by a black hood and whose entire body is concealed by a black costume is called a Kurogo. In Kabuki, there is a theatrical convention that black is invisible, so Kurogo are considered to be invisible on stage.
Kurogo perform various tasks, but basically make it easy for actors to play their roles, mainly by handing over or putting away props, and by helping with costume changes. If a Kurogo stands out on stage, it interferes with the performance, so a Kurogo enters quickly without making any noise, and conceals himself behind an actor or a stage set item such as a tsuitate (small screen) to remain as invisible as possible and does his work. If the timing of the Kurogo's movements and the actor's timing are not in harmony, this interferes with the action on stage, so in many cases the actor's apprentices take the parts of Kurogo.
If a Kurogo wears a black costume in a snow scene or sea scene, the black costume will stand out too much, so he sometimes changes his black costume for a white costume or light blue costume. In these cases, he is called a Yukigo or Mizugo.