Cannot use hex strings or named matplotlib colors with DataFrame.plot
I was going to put this on SO but I think it might belong here instead:

I have a pandas DataFrame where I want to plot each column as a line with the same color. This might sound useless but I am showing groups of lines from certain selections of the DataFrame. The only problem is it appears that the `DataFrame.plot` method does not accept any matplotlib hex strings or named colors in the color kwarg.

A trivial example:

```
pd.DataFrame([range(10),range(10)]).plot(color='#008000')
```

gives a blank subplot with no errors, however, 

```
pd.DataFrame([range(10),range(10)]).plot(color='g')
```

gives the expected result. To me this seems like a bug considering that `color='g'` works but not its hex string equivalent. The same issue occurs when trying to use a named matplotlib color like `color='lightgray'`. This does however work when using `Series.plot` or `DataFrame.plot` with one column or if a list of hex strings or named colors are given with length equal to the number of columns as in,

```
pd.DataFrame([range(10),range(10)]).plot(color=['#008000']*2)
```
