code: function _setColourOption (key, val) {
    if (is.object(val) || is.string(val)) {
      const colour = color(val);
      this.options[key] = [
        colour.red(),
        colour.green(),
        colour.blue(),
        Math.round(colour.alpha() * 255)
      ];
    }
  }
docs: Update a colour attribute of the this.options Object.
  @private
  @param {String} key
  @param {String|Object} val
  @throws {Error} Invalid key