As far as I know, "it's getting colder" is 寒くなってきた.
The way I'm understanding this is you're saying that the state of the weather becoming colder has come, similar to how something might "come to pass" in English.
What confuses me is that this expression already uses くる in the past tense, so how would you phrase "it was getting colder"? Do I need some sort of temporal noun or reference to a past time period, like "last month"?
寒くなってきた is surely a phrase for a situation when English speakers would say "it's getting colder", but that doesn't mean that they are exactly the same. So, it's not necessarily constructive to develop it.
As you might know, "was -ing" can be expressed with ていた if the conjugated verb is durative one. In this regard, 寒くなる is, however, an instantaneous verb, in other words, 寒くなっていた only means "it had gotten cold".
So, you have to change it into a durative verb like 寒くなってくる or 寒くなっていく, then, conjugate it into ていた form of each of them, i.e. 寒くなってきていた and 寒くなっていっていた (rare).
Besides, you can use 寒くなりつつあった too.
I guess "It was getting colder" (unlike it's getting colder) would not be a statement (without context), but more like (at least often) relating to what happened at the time when it got colder.
If so, e.g. 寒くてってきたごろ。。。
