Setting the color or a Circle patch directly is not supported through class methods as we might assume;
circle.set_color('b')
^^ this is incorrect.
Patches have two distinct colors, facecolor and edgecolor, which you can set by, respectively c.set_facecolor(...) (or set_fc) and c.set_edgecolor(...)(or set_ec).
The setp machinery is just a convince method for looking up these functions. setp(artist, key=val) does, more-or-less getattr(artist, "set_%s" % key)(val).
facecolororedgecolor.