The motivation is explained in the changelog entry.

Note that even though

plt.plot([1, 2], [3, 4], "Y")

"worked" so far,

plt.plot([1, 2], [3, 4], "Yo")

(for example), i.e. additionally passing a marker spec, has not worked
at least since matplotlib 1.5 (a ValueError is thrown). As this does
not appear to have ever been raised on the issue tracker, I believe the
number of users of UPPERCASE colors must be limited.

(noted while working on #13210, where I had to use "Y0" instead of "Y"...)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way