[BUG] KDE plot and color kwarg
Not sure if this is a bug here or not.

```
import pandas as pd
import numpy as np

d = {'noise': np.random.randn(100).cumsum(),
     'trend': np.arange(100)}

df = pd.DataFrame(d)
df['resid'] = pd.ols(y=df.noise, x=df.trend).resid
df['resid'].plot(kind='kde', color='k')
```

Will give this traceback:

```
In [14]: Traceback (most recent call last):
      File "/usr/local/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
        draw(artist, renderer, *args, **kwargs)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/figure.py", line 927, in draw
        func(*args)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
        draw(artist, renderer, *args, **kwargs)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/axes.py", line 1999, in draw
        a.draw(renderer)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
        draw(artist, renderer, *args, **kwargs)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/lines.py", line 494, in draw
        gc.set_foreground(self._color)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/backends/backend_macosx.py", line 189, in set_foreground
        GraphicsContextBase.set_foreground(self, fg, isRGB)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 878, in set_foreground
        self._rgb = colors.colorConverter.to_rgba(fg)
      File "/usr/local/lib/python2.7/site-packages/matplotlib/colors.py", line 354, in to_rgba
        raise ValueError('to_rgba: Invalid rgba arg "%s"\n%s' % (str(arg), exc))
    ValueError: to_rgba: Invalid rgba arg "['k']"
    need more than 1 value to unpack

    If you suspect this is an IPython bug, please report it at:
        https://github.com/ipython/ipython/issues
    or send an email to the mailing list at ipython-dev@scipy.org

    You can print a more detailed traceback right now with "%tb", or use "%debug"
    to interactively debug it.

    Extra-detailed tracebacks for bug-reporting purposes can be enabled via:
        %config Application.verbose_crash=True
```

I haven't add a chance to look through the plotting code yet to see if this should work or not, but it seems like it should.

```
In [14]: pd.__version__
Out[14]: '0.11.0.dev-7d11531'
```

And this happens in IPython both with and without pylab mode.  I've tried with other rgba values and they seem to fail too.
