I just realized I have at least four different story-identification questions involving the Tandy TRS-80 Color Computer. Does it make sense to add a tag for it? If so, ought it to be color-computer? tandy-color-computer? trs-80?
I don't think it matters either way but without a good case that we do need it I'd err on the side of not creating it.
When should I create new tags?
Most common tags already exist on a mature site. You should always favor existing tags; only create new tags when you feel you can make a strong case that your question does cover a new topic that nobody else has asked about before on this site.
Currently these questions are covered under story-identification video-games, I'm not sure a trs-80-color-computer would add much value on top of that. Besides if you're mentioning it in the text you're not really increasing it's "searchability" only the categorisation.
I think it could be a useful tag, so I'd err on the side of creating it.
Like I've said before, a tag is a Bat-signal for experts. And expert is quite broad here: professional experts, avid fans, creators of content, and so on.
In this case, you're trying to identify a work that was published on a given platform, that platform being the Tandy TRS-80 Color Computer. You're likely to get an answer from someone who is an expert — in some fashion — on the Tandy TRS-80 Color Computer.
How would you attract such an expert? By tagging your question with tandy-color-computer or a similar suitable tag.