How to pick a new color for each plotted line within a figure in matplotlib?

I don't know if you can automatically change the color, but you could exploit your loop to generate different colors:

for i in range(20):
   ax1.plot(x, y, color = (0, i / 20.0, 0, 1)

In this case, colors will vary from black to 100% green, but you can tune it if you want.

See the matplotlib plot() docs and look for the color keyword argument.

If you want to feed a list of colors, just make sure that you have a list big enough and then use the index of the loop to select the color

colors = ['r', 'b', ...., 'w']

for i in range(20):
   ax1.plot(x, y, color = colors[i])

prop_cycle

color_cycle was deprecated in 1.5 in favor of this generalization: http://matplotlib.org/users/whats_new.html#added-axes-prop-cycle-key-to-rcparams

# cycler is a separate package extracted from matplotlib.
from cycler import cycler
import matplotlib.pyplot as plt

plt.rc('axes', prop_cycle=(cycler('color', ['r', 'g', 'b'])))
plt.plot([1, 2])
plt.plot([2, 3])
plt.plot([3, 4])
plt.plot([4, 5])
plt.plot([5, 6])
plt.show()

Also shown in the (now badly named) example: http://matplotlib.org/1.5.1/examples/color/color_cycle_demo.html mentioned at: https://stackoverflow.com/a/4971431/895245

Tested in matplotlib 1.5.1.

matplotlib 1.5+

You can use axes.set_prop_cycle (example).

matplotlib 1.0-1.4

You can use axes.set_color_cycle (example).

matplotlib 0.x

You can use Axes.set_default_color_cycle.

I usually use the second one of these:

from matplotlib.pyplot import cm
import numpy as np

#variable n below should be number of curves to plot

#version 1:

color = cm.rainbow(np.linspace(0, 1, n))
for i, c in zip(range(n), color):
   plt.plot(x, y, c=c)

#or version 2:

color = iter(cm.rainbow(np.linspace(0, 1, n)))
for i in range(n):
   c = next(color)
   plt.plot(x, y, c=c)

Example of 2: