I don't really use pie charts, but I think it just uses the default color cycle. This can be altered by changing the rcParams:
>>> import matplotlib.pyplot as plt
The color_cycle parameter is just a python list, so I use list.remove to remove black (which is the letter 'k' since 'b' is blue). There are other ways of setting rcParams, as detailed in the help files
(Note that `rc` and `rcParams` is in both matplotlib.pyplot and the main matplotlib package).