Using matplotlib master and a color defined by a string indicating a gray shade, the alpha parameter gets ignored in calls to, e.g. plt.plot. Example:

intercepts=np.random.randn(100)

ms=np.random.randn(100)
x=np.linspace(-5,5,100)
for m,i in zip(ms,intercepts):
    plt.plot(m*x+i, c='0.1', alpha=0.2)

results in:

whereas the expected result can be obtained by converting the string to a tuple:

for m,i in zip(ms,intercepts):
    plt.plot(m*x+i, c=(0.1, 0.1, 0.1, 1.0), alpha=0.2)

Looking at the code, it seems that all colors are passed through colors.to_rgba, and in my setup it correctly converts the string to the tuple, so it seems the problem is elsewhere.

>>> mpl.colors.to_rgba('0.1')
(0.1, 0.1, 0.1, 1.0)

This used to work in previous v2.x branch commits (cannot really pinpoint where it failed first time) and in v1.5.