A question is opinion-based when, in a nutshell, the answer is "whatever you want the plot to do". For example, a question where the answer is dependent on the story character's motivations and feelings, meaning that there are an infinite number of equally possible answers depending on the person's mood and backstory.
But there's a pretty big exception that arises when you are asking not about one, but a large quantity of people making a motivated decision. For example, "Why would a firefighter save his pet kitten over his neighbours?" would get closed for opinion-based with the fury of a thousand suns. But "Why would a society value pets over people?" wouldn't - at least not for being opinion-based.
Questions about people in aggregate ask about not individual motivations, but about societal circumstances. And the scope does not have to be an entire culture. It can be limited to an occupation, as here:
- Why would an adventurer use a sword frog? (why would [occupation] do X)
- The eating habits of metabolic wizards (how should [occupation] do X)
- How can an absolute ruler in a large country keep their regional governors/politicians in check? (how could [occupation] do X)
The key factor is that when you are asking about many people, the individual's motivations are not assumed to apply - or only in the broadest sense. An individual can decide anything based on their preferences, but the preferences of a group of people are a property of the world; and therefore a part of world-building.
That, that is my understanding of the matter. Please correct me if I am wrong. Since if I understand it, then surely, this question should have been kept open? It asks about what people would name colonies. Sure, an individual can name their colony Mittenstown because they love their cat so much, but colonisers, in aggregate, form an occupation. So what names would be used for colonies, that sounds very much like a question about the rules of a system to me. Therefore, not opinion-based.
But if I made any mistake in this reasoning then I would love to be corrected. I don't care as much about this question as I do about understanding WB.SE's rules; a task that has been challenging me to this day.