According to official documentation below code snippet should return a plot with red lines
Documentation is here

import matplotlib as mpl
import matplotlib.pyplot as plt
data = np.random.randn(50)
mpl.rcParams['lines.color'] = 'r'
plt.plot(data)

Actual outcome

Expected outcome

Matplotlib version

• Operating system: Windows 10
• Matplotlib version: 3.1.2
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 3.7.3