Bug report

Bug summary

2-D array color does not work in plt.plot() while working as intended in plt.scatter(). A 2-D array is recommended by matplotlib itself by this warning:
"'c' argument looks like a single numeric RGB or RGBA sequence, which should be avoided as value-mapping will have precedence in case its length matches with 'x' & 'y'. Please use a 2-D array with a single row if you really want to specify the same RGB or RGBA value for all points."

Code for reproduction

import matplotlib.pyplot as plt
import numpy as np

color_rgb = np.array([[0.1,0.2,0.3]])
color_rgba = np.array([[0.1,0.2,0.3,0.4]])

plt.scatter([1,2],[1,2],c=color_rgb)  # working
plt.plot([1,2],[1,2],c=color_rgb)     # not working

plt.scatter([1,2],[1,2],c=color_rgba) # works fine
plt.plot([1,2],[1,2],c=color_rgba)    # NOT working

plt.show()

Actual outcome

Invalid RGBA argument: array([[0.1, 0.2, 0.3, 0.4]])

Matplotlib version

• Operating system: Win 10 1909
• Matplotlib version: 3.1.3 (from anaconda, pkgs/main)
• Matplotlib backend (print(matplotlib.get_backend())): module://ipykernel.pylab.backend_inline
• Python version: 3.8.1