Ryuko, a coacher of the company, colored her tooth in black in her stage performance.  That is called "O-Haguro".  
"O-Haguro" was founded in the 8th century during the nobles and developed in the Edo period during the married women.  "O-Haguro" is today an extinct habit.  Most actresses don't make "O-Haguro", even if they play Edo-women roles on stage.  She is professional !!