Let's say I have a revision that I've configured not to receive any traffic, but the revision is tagged with the tag "green". The revision is also configured to always have at least one instance. Am I charged for this build, because the "tag" makes it into a "serving" revision (as mentioned in these documents)?
Yes, it's like a king of trap!! If you add a tag on a revision, you make it up and ready to receive traffic, even if it's only traffic to a specific URL and not a routing through the Cloud Run load balancer.
And because the revision is up and running to serve traffic you will pay for the min instance.