The usual way to set the line color in matplotlib is to specify it in the plot command. This can either be done by a string after the data, e.g. "r-" for a red line, or by explicitely stating the color argument.
import matplotlib.pyplot as plt
plt.plot([1,2,3], [2,3,1], "r-") # red line
plt.plot([1,2,3], [5,5,3], color="blue") # blue line
plt.show()
See also the plot command's documentation.
In case you already have a line with a certain color, you can change that with the lines2D.set_color() method.
line, = plt.plot([1,2,3], [4,5,3], color="blue")
line.set_color("black")
import matplotlib.pyplot as plt
import pandas as pd
df = pd.DataFrame({ "x" : [1,2,3,5], "y" : [3,5,2,6]})
df.plot("x", "y", color="r") #plot red line
plt.show()
If you want to change this color later on, you can do so by
plt.gca().get_lines()[0].set_color("black")
This will get you the first (possibly the only) line of the current active axes.
In case you have more axes in the plot, you could loop through them
for ax in plt.gcf().axes:
ax.get_lines()[0].set_color("black")
and if you have more lines you can loop over them as well.