Bug summary

The "standard" matplotlib color semantics is that the strings "none" and "None" both mean fully transparent (as can be checked by mpl.colors.to_rgba); the object None can sometimes be used to mark a fallback. Because the matplotlibrc format cannot distinguish between None and "None" (due the use of unquoted strings), it normally uses "auto" or "inherit" to mark a fallback.
However, rcParams["legend.labelcolor"] (a relatively recent introduction -- matplotlib 3.5) tries to use "None" as default, and as a result ends up having inconsistent behavior between "None" and "none".

Code for reproduction

from pylab import *

rcdefaults()
print(repr(rcParams["legend.labelcolor"]))
# default is the string "None"

rcParams["legend.labelcolor"] = "None"; plot([0, 1], label="foo"); legend()
# labels are black

rcParams["legend.labelcolor"] = "none"; plot([0, 1], label="foo"); legend()
# labels are transparent

rcParams["legend.labelcolor"] = None; plot([0, 1], label="foo"); legend()
# labels are black

Actual outcome

See above.

Expected outcome

"None" and "none" should mean the same (likely both should mean "transparent", not so much for usefulness but rather for consistency.

Additional information

I suspect trying to support the object None as well here basically cannot be made to work (as long as matplotlibrc uses unquoted strings); the default should switch to be "auto" (or "inherit").

As a further point, the implementation of the rcParams validator _validate_color_or_linecolor seems wrong:

    elif isinstance(s, str) and len(s) == 6 or len(s) == 8:  # (1)
        stmp = '#' + s
        if is_color_like(stmp):
            return stmp
        if s.lower() == 'none':  # (2)
            return None

Likely parentheses are missing at (1); also (2) can never hold due to the len() check.
As a side point, 3 and 4-hex color codes ("abc(d)", meaning "#abc(d)", meaning "#aabbcc(dd)") are also not supported by rcParams (this is also the case for the more general _validate_color), even though the normal color machinery does support them. At least this should be documented; I don't know if this can actually be supported without introducing an ambiguity with named colors.

Operating system

macos

Matplotlib Version

3.9.0.dev1542+geb62d6951a

Matplotlib Backend

any

Python version

3.12

Jupyter version

no

Installation

git checkout