/**
     * 计算反色,
     * @param {*} a 色值
     * @param {*} ilighten 减弱对比度(-1 ~ -15)
     * @returns
     * 示例: oppositeColor("#000000", -4); 返回: #bbbbbb
     */
oppositeColor(a, ilighten) {
        a = a.replace('#', '');
        //var max16 = 15;
        var max16 = Math.floor(15 + (ilighten || 0));
        if (max16 < 0 || max16 > 15) max16 = 15;

        var c16,
            c10,
            b = [];

        for (var i = 0; i < a.length; i++) {
            c16 = parseInt(a.charAt(i), 16); // to 16进制
            c10 = parseInt(max16 - c16, 10); // 10进制计算
            if (c10 < 0) c10 = Math.abs(c10);
            b.push(c10.toString(16)); // to 16进制
        }
        return '#' + b.join('');
    }