code: function setDefault(levels, ecModel) {
      var globalColorList = ecModel.get('color');
  
      if (!globalColorList) {
          return;
      }
  
      levels = levels || [];
      var hasColorDefine;
      zrUtil.each(levels, function (levelDefine) {
          var model = new Model(levelDefine);
          var modelColor = model.get('color');
  
          if (model.get('itemStyle.color')
              || (modelColor && modelColor !== 'none')
          ) {
              hasColorDefine = true;
          }
      });
  
      if (!hasColorDefine) {
          var level0 = levels[0] || (levels[0] = {});
          level0.color = globalColorList.slice();
      }
  
      return levels;
  }
docs: set default to level configuration