Translation Issues.

Comparing the Greek text of both verses you listed above, (Col. 1.15 and Jn 1.18) they are similar.
This is as you note, a translation issue rather than a textual issue.

(1) In the first instance the NIV is simply supplying "Son" from the context of verse 13 to assist the reader to understand to whom the verse is referring.

Other versions do this as well.
I don't like the practice but do not think it is completely out of line.
In fact, one of the differentiating points between the textus receptus and the modern critical text is this very feature,
the TR is accused of adding or clarifying the text.​

(2) Why are you reading the New World Translation (NWT), it's a Jehovah's Witness Translation.

Jehovah's Witnesses do not believe in the deity of Christ.
That is why the word "the only-begotten god" is not capitalized.​

Rob

 

Yes, the word “over” is not found in verse 15 of Colossians,
but keep reading, it is conveyed in the rest of the passage.

who is the invisible God’s image,
all Creation’s Firstborn,
because in him all things were created,
in the heavens and on the land,
the visible and the invisibles
whether thrones or lordships
whether leaders or authorities,
all thing were created through him and for him.
He is before all things
and in him all things co-exist.
He is the head of the Body, the assembly.
Firstborn from among the dead ones,
so he might in all things become the preeminenting one.
Because in him all the fullness was delighted to reside,
and through him to reconcile all things to him,
making peace through his cross's blood,
through him,
whether things on the land
or things in the heavens.
Colossians 1:15-20
Scott McKnight, The Second Testament, IVP Academic, 2023​

Rob