tiny dataframe non-string colors bug
Hi,

If we specify custom rgb 3-ples for rcParams['axes.color_cycle'], DataFrame.plot breaks because  _get_colors expects strings (the usual rcParams['axes.color_cycle'] = ['b','g'...] etc).

Test case:
rcParams['axes.color_cycle'] = [(1,1,1),(.1,.1,.1)]
DataFrame({'a':[1,1], 'b':[2,2]}).plot()

Seems like having _get_color return a list instead of taking the list and joining all the elements would fix it?

Thanks!
