code: function updatePen( penIndex, penColor = "0" ) {
      if ( !this._content ) return;
  
      let elementsToFill = this._content.elementsToFillPerColor[ penIndex ];
      if ( !Array.isArray( elementsToFill ) ) elementsToFill = [];
      let elementsToStroke = this._content.elementsToStrokePerColor[ penIndex ];
      if ( !Array.isArray( elementsToStroke ) ) elementsToStroke = [];
  
      updateColor( elementsToFill, elementsToStroke, penColor );
  }
docs: Update the color of a pen.
  
  @this ViewXJS
  @param   {integer} penIndex - The index of the pen.
  @param   {string} penColor - The new color of this pen.
  @returns {undefined}