If a package is marked red, it is either not available from any of the repoes currently active, or a package is made ready to be reverted back to a previous version. Packman does downgrade packages from time to time, I suppose it has to do with a failure/inconsistency in the most recent version, not recognised initially? Packman does also remove certain packages to be replaced by new ones, in that case it will be as in the first red example; although it derives from Packman (given that the packman repo is active) it is not available as it does not any longer exist.
If the package libx264-125 isn’t available no more just remove it, the dependency solver should auto-correct the matter and the package should be replaced. If an older version is available just downgrade.
Typically this package updates numerically and leaves behind the old < number.
The old package can be removed, but it's not harmful to leave it either.
View the versions in software manager, theat should make it clear