How to interpret red text in Yast repo tab

Every so often there is a software update which leaves some packages listed in the repositories-Installed/Available section in red text.
I take it this means a problem of some kind.
For example I have a Skype package “borrowed” from another SUSE distro on my 13.2 system which works, but is in a sense foreign. This I understand.
Recently an update installed some systemd packages and now they list red in Yast.
eg systemd … 210-25.19.1 (210-25.16.1) # in red text
These are critical packages which I do not experiment with under any circumstances.
Am I right in assuming that this marking is simply providing me with a path back to the previous version if there is a problem?

In Yast----Software a red marked package means:
There is only a same Package with lower Version (Number) avaible in the Repos as the installed one.
The installed Package-Version is not (more) avaible in the Repos.

Not necessarily.
Packages marked in red can mean one of two things:

  • this package does not exist in any configured repo
  • the package you installed is newer/has a higher version than available in any repo

For example I have a Skype package “borrowed” from another SUSE distro on my 13.2 system which works, but is in a sense foreign. This I understand.

Yes, it is not available in any repo, and therefore marked as red.

Recently an update installed some systemd packages and now they list red in Yast.
eg systemd … 210-25.19.1 (210-25.16.1) # in red text

There was an update to systemd, but it was retracted because of (possible) problems.
https://bugzilla.opensuse.org/show_bug.cgi?id=948555

You installed the update, but as it has been removed, you have a higher version now than available in the repos.
If you don’t have problems, you can just ignore it. The next update will “fix” that, as it will have a higher version number.
Or downgrade those packages manually (via “Versions” below the package list, or simply choosing “Update Unconditionally”).

Am I right in assuming that this marking is simply providing me with a path back to the previous version if there is a problem?

No.
In the case of official updates in the update repo, there’s always a “path back to the previous version”, as all versions are kept in the repo.
For “normal” repos, this is not true. Only the latest version is available there.