It seems you may have omitted the very meat of the message Jesus, before becoming a man, sent Isaiah to show us. Omitting scriptures that might change the tradition or doctrine that one might be pushing is common, and many do this without even thinking about it. Let's include the rest of what Jesus told Isaiah.
Is. 1:
2 Hear, O heavens, and give ear, O earth: for the LORD hath spoken, I have nourished and brought up children,
and they have rebelled against me.3 The ox knoweth his owner, and the ass his master's crib:
but Israel doth not know, my people doth not consider.
4 Ah sinful nation, a people
laden with iniquity, a
seed of evildoers, children that are corrupters: they have forsaken the LORD, they have provoked the Holy One of Israel unto anger, they are gone away backward.
It seems these people were already in disobedience and dishonor to God and His Instructions.
10 Hear the word of the LORD, ye
rulers of Sodom;
give ear unto the law of our God, ye
people of Gomorrah.
11 To what purpose
is the multitude of your sacrifices unto me? saith the LORD: I am full of the burnt offerings of rams, and the fat of fed beasts; and I delight not in the blood of bullocks, or of lambs, or of he goats.
12 When ye come to appear before me, who hath required this at your hand, to tread my courts?
13 Bring no more vain oblations; incense is an abomination unto me; the new moons and sabbaths, the calling of assemblies, I cannot away with;
it is iniquity, even the solemn meeting.
14 Your new moons and your appointed feasts my soul hateth: they are a trouble unto me; I am weary to bear
them.
15 And when ye spread forth your hands,
I will hide mine eyes from you: yea, when ye make many prayers,
I will not hear: your hands are full of blood.
What good is it to partake in Passover and Feast of Unleavened Bread if there is no real repentance and effort to obey?
16 Wash you, make you clean; put away the evil of your doings from before mine eyes;
cease to do evil;
17 Learn to do well; seek judgment, relieve the oppressed, judge the fatherless, plead for the widow.
These, like the Pharisees, has omitted the "weightier matters of the Law" like judgment and mercy. They should have done these, and not to leave the other undone.
18 Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.
19 If ye be willing and obedient, ye shall eat the good of the land:
Jesus repeated this very sentiment in John 14.
23 Jesus answered and said unto him, If a man love me,
he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.
24 He that loveth me not
keepeth not my sayings: and the word which ye hear is not mine, but the Father's which sent me.
Is. 1:26 And I will restore thy judges as at the first, and thy counsellors as at the beginning: afterward thou shalt be called, The city of righteousness, the faithful city.
27
Zion shall be redeemed with judgment, and her converts with righteousness.
Is. 2:2 And it shall come to pass in the last days, that the mountain of the LORD'S house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it.
3 And many people shall go and say, Come ye, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem.
Is. 66:
21 And I will also take of them for priests
and for Levites, saith the LORD.
22 For as the new heavens and the new earth, which I will make, shall remain before me, saith the LORD, so shall your seed and your name remain.
23 And it shall come to pass, that from one new moon to another, and from one sabbath to another,
shall all flesh come to worship before me, saith the LORD.
So to use a couple sentences from the middle of one chapter in Isaiah to preach that God's "Feasts" are Pagan or as you imply in Col. 2,"rudiments of the World" doesn't fit with the rest of Isaiah's prophesies, or the entire teaching of the Law and Prophets for that matter.
I understand the battle against the Word of God which started with the serpent using parts of God's Word to deceive Eve.
Rev. 14:
17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed,
which keep the commandments of God,
and have the testimony of Jesus Christ.
I truly hope you might consider this battle and not be swept away with it.