Again, when the 7th is is based on when God rested after Creation, not on when people feel like starting to count. Different calendars organize the months differently, but counting off every 7th day since Creation is the same regardless of which calendar system is being used.
In Acts 17:11, the Bereans were praised because they diligently tested everything that Paul said against OT Scripture to see if what he said was true, so Paul should not be interpreted as saying things that they would have outright rejected. For example, in Deuteronomy 4:2, it prohibits adding to or subtracting from God's law, and in Deuteronomy 13:4-5, the way that God instructed His people to determine that someone was a false prophet who was not speaking for Him was if they taught against obeying His law, so if that had been what Paul was doing in Colossians 2:16, then according to God, we should disregard what Paul said as being the words of a false prophet. So if you affirm that Paul is a servant of God who is speaking for Him and you interpret what he said as speaking against following what God has commanded as if he were not a servant of God, then you should be quicker to think that you must have misinterpreted what he said than to think that your interpretation is correct.
In regard to Colossians 2:16, it is ambiguous in itself whether Paul was saying not to let anyone judge them for obeying what God has commanded or for not obeying what God has commanded. In other words, it is possible that they were not doing what God has commanded, they were being judged by Jews for not doing that, and Paul was saying not to listen to them, or it is possible that they were obeying what God has commanded, they were being judged by pagans because they were doing that, and Paul was encouraging them not to let anyone judge them and keep them from obeying God. In Colossians 2:20-23, they were being judged by those teaching human precepts, self-made religion, asceticism, and severity to the body, so they were being judged by pagans, which means that they were being judged because they were obeying what God has commanded and Paul was encouraging them not to listen to them.