let hexString = '#000000', hexString2 = '#ffffff';
let arr1 = hexString.substring(1, hexString.length).match(/.{1,2}/g).map(item => parseInt(item, 16)),
    arr2 = hexString2.substring(1, hexString.length).match(/.{1,2}/g).map(item => parseInt(item, 16));

let res = arr1.map(function (item, i) {
    let arr = [];
    if (item < arr2[i]) {
        for (var k = item; k <= arr2[i]+1;) {
            arr.push(Math.round(k));
            k += (arr2[i] - item) / 16;
        }
    }
    else{
        for (var m = item; m>= arr2[i];) {
            arr.push(Math.round(m));
            m-=(item - arr2[i])/16;
        }

    }
    return arr;
});