docs: function(x) {
          if (!x.rgba) {
              return -1;
          }
          return (x.rgba[0] === this.rgba[0] && x.rgba[1] === this.rgba[1]
                  && x.rgba[2] === this.rgba[2] && x.rgba[3] === this.rgba[3]) ? 0
                  : -1;
      }