Nothing in the book of Numbers or Jeremiah tells me that Paul's letter to the Colossians is to be treated as scriptural but that Shepherd of Hermas is not. Neither even mentions an additional set of writings that are to be given the status of 'scripture'.
Come to think of it, when 'the scriptures' appears in the book of Acts and 'scripture' appears in the second epistle of Timothy, nothing there suggests that they are referring to anything additional to the Hebrew scriptures.
At the time of Acts unfolding, Paul writing letters, Peter adding, Luke and Mark contributing, James & Matthew submitting theirs, written scriptures were still the Hebrew Bible. In the days of Jesus and beyond some help was available in the Greek Septuagint translation of the Hebrew texts. That's all there was, those two sources. The apostle's letters required many decades of copying to distribute to churches under extreme persecution and punishment for distributing the word of God. So the "scriptures" in Acts and Timothy referred to the OT. Then we find Peter was the first to refer to epistles of Paul's in
2 Peter 3:15-16 (KJV)
15 And account
that the longsuffering of our Lord
is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you;
16 As also in all
his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as
they do also the other scriptures, unto their own destruction.
Peter and Paul both relied heavily on scriptures of the Torah and Tanach, and so did Jesus. All Jesus had was the Hebrew Bible and of course what the Father supplied. He left the Holy Spirit in charge of adding further knowledge of the word of God. It's all interactive.
The Colossian letter dealt with the fact that Jesus fulfilled the requirements of the Law, answered the prophets, and warned of the consequences of returning to the Law. Jews were plaguing gentiles, demanding adherence to Moses. God warned Israel long before Jesus came that he would dismantle their nation and ways until the new millennium, when Israel will be restored in the proper glory with Jesus on the throne. Much of the NT makes little sense without knowledge of the old way, but that knowledge isn't required for a gentile to be saved.