ohh, got it! Thank you! 
I missed accept: all. My bad, sorry.
I got it to work for querying/adding basic string tags.
project_tags:
type: tags
accept: all
options:
type: query
query: page.siblings.pluck("project_tags", ",", true)
Still, do you think you can point me into a direction for how to go about querying and adding structured items to a global list from within another page, for example a tag with an associated color, or let’s say a name with an associated external url.
Adding and querying tags with colors to/from a globally accessible list is possible in Notion and GitHub issues. I’m a little lost in what to even search for in the docs. I’d assume a custom field or blueprint, or page methods as you describe here?:
Or maybe this is not possible and I’m missing something fundmental about Kirby here?