[MNT]: Inconsistent use of "inherit" and "auto" for color fallback in rcParams

[anntzer]

Summary

Some color rcParams that can fallback to another rcParam use "auto" to indicate that fallback (lines.markerface/edgecolor, boxplot.flierprops.markerfacecolor, axes.titlecolor, savefig.face/edgecolor), whereas others use "inherit" (legend.face/edgecolor, x/ytick.labelcolor).

There's also legend.labelcolor, which uses "None", but that's a much more complicated case (#28153).

Proposed fix

This should be made consistent. Safest (and not really an issue) would be to just support "auto" and "inherit" with the same meaning for all those rcParams; it would also be possible to pick a single winner, use it consistently, and deprecate usage of the other, although I'm not really sure it's worth the annoyance.

[story645]

it would also be possible to pick a single winner, use it consistently, and deprecate usage of the other, although I'm not really sure it's worth the annoyance.

pick a winner and silently support the other? I think for reading code or new commits, it's easier if there's only one consistently used throughout the codebase (I think inherit is more descriptive) but agree that deprecation probably isn't worth it.

[anntzer]

I would prefer auto, agreeing with the reasoning at #15111 (comment). But sure, I guess we could switch places using inherit to use auto and keep undocumented support for inherit there.

[story645]

keep undocumented support for inherit there.

Forgot in my first comment, but that is one of the reasons we have the discouraged admonition. I'll defer to you on auto being the winner.