from pylab import *

plt.plot([1, 2], "C0")
plt.plot([2, 3], c="C0")
rcParams["axes.prop_cycle"] = cycler(color=["r", "g", "b"])
show()

The first call to plot() resolves the color eagerly, the second one resolves it lazily (which I think is the intended behavior).

mpl master (~3.0.x)