I just posted a rainbow related question and thought that a rainbow tag might be useful given that there are a few questions about it. Didn’t want to create one without consulting meta though.
Good question, but I don't think rainbows constitute a specific area of expertise that is worth highlighting with a tag.
Questions about rainbows that are on topic on this site should fall under electromagnetic-radiation or perhaps visible-light, and possibly also optics; maybe there's another tag or two along those lines that I can't think of right now. Anyway, those tags identify the areas of knowledge that are relevant to answering questions about rainbows. Between those tags and the title of the question, people will have a pretty good idea of whether they can answer it or might be interested in reading it, and I don't think adding rainbows would make much difference.
It probably wouldn't hurt to have the tag either, but we've got a lot of tags already and I'd vote against adding tags unless they really contribute significant value. That being said, if you have enough reputation to create a new tag and you really think one is needed, you can always just go ahead and do it. Checking with meta first is definitely appreciated but in no way required.