BUG: df.plot.scatter may cause UserWarning: No data for colormapping provided via 'c'. Parameters 'cmap' will be ignored

[GianlucaFicarelli]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame({"x": [10, 11, 12, 13], "y": [20, 21, 22, 23]})
fig, ax = plt.subplots(nrows=1, ncols=1)
df.plot.scatter(x="x", y="y", c="b", ax=ax)

Printed warning:

venv/lib/python3.11/site-packages/pandas/plotting/_matplotlib/core.py:1256: UserWarning: No data for colormapping provided via 'c'. Parameters 'cmap' will be ignored
  scatter = ax.scatter(

Issue Description

If the color b (blue) is passed as parameter c to df.plot.scatter, a UserWarning is printed.

The UserWarning isn't printed when passing c='blue' or c=['b'].

Looking at the code, this is caused by

pandas/pandas/plotting/_matplotlib/core.py

Lines 1237 to 1240 in 6b7225a

	elif is_integer_dtype(c_values):
	# pandas uses colormap, matplotlib uses cmap.
	cmap = "Greys"
	cmap = mpl.colormaps[cmap]

that is causing cmap to be set, because is_integer_dtype('b') is True, since pandas_dtype('b') is int8.

Expected Behavior

The UserWarning shouldn't be generated when c='b', since 'b' is a valid color.

Installed Versions

INSTALLED VERSIONS

commit : 965ceca
python : 3.11.1.final.0
python-bits : 64
OS : Darwin
OS-release : 21.6.0
Version : Darwin Kernel Version 21.6.0: Mon Apr 24 21:10:53 PDT 2023; root:xnu-8020.240.18.701.5~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : en_US.UTF-8
LANG : None
LOCALE : en_US.UTF-8

pandas : 2.0.2
numpy : 1.25.0
pytz : 2023.3
dateutil : 2.8.2
setuptools : 68.0.0
pip : 23.1.2
Cython : 0.29.35
pytest : 7.3.2
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.14.0
pandas_datareader: None
bs4 : 4.12.2
bottleneck : None
brotli : None
fastparquet : 2023.4.0
fsspec : 2023.6.0
gcsfs : None
matplotlib : 3.7.1
numba : None
numexpr : 2.8.4
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 12.0.1
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.10.1
snappy : None
sqlalchemy : None
tables : 3.8.0
tabulate : None
xarray : 2023.5.0
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[jlrodriguezp]

The UserWarning isn't printed when passing c=['cool'] or c=['b'].