The following code illustrates the problem:

import matplotlib.pyplot as plt

plt.subplot(211)
plt.plot([2, 1], c='k')
plt.plot([1, 2])

plt.subplot(212)
plt.plot([2, 1], color='k')
plt.plot([1, 2])

One would expect the same plot, but actually the index of color_cycle is increased when using 'c', and stays the same when using 'color'. The latter is imo the right behavior. Matplotlib 1.4.3.