_dataframe.boxplot_ with _where_ and _by_ does not respect color keyword
### Bug report

**Bug summary**

The boxplot method on a dataframe which is using the "column, by" keywords does
not respect the _color_ keyword, and in fact crashes if it is present. This is not consistent with the documentation [here](http://pandas.pydata.org/pandas-docs/stable/user_guide/visualization.html#box-plots).

**Code for reproduction**

```python
import pandas as pd
import matplotlib.pyplot as plt
import numpy as np

def make_dummy_data():
    """ Return """
    df1 = pd.DataFrame(np.random.rand(10, 3), columns = ['x', 'y', 'z'])
    df2 = pd.DataFrame(2*np.random.rand(10, 3), columns = ['x', 'y', 'z'])
    return df1, df2

def comparative_results():
    """ stuff """

    df1, df2 = make_dummy_data()

    def draw_plot(ax, data, edge_color, fill_color=None):
        """ Controls details of color"""
        colors = dict(boxes=edge_color, whiskers=edge_color, medians=edge_color, caps=edge_color)
        ax = data.boxplot(column=['x'], by=['z'], showfliers=False, ax=ax, color=colors)
        return ax

    ax = None
    ax = draw_plot(ax, df1, 'k')
    ax = draw_plot(ax, df2, 'r')
    ax.set_title('dummy to expose bug')
    plt.show()


if __name__ == "__main__":
    comparative_results() 
```

**Actual outcome**

<!--The output produced by the above code, which may be a screenshot, console output, etc.-->

```
Traceback (most recent call last):
  File "/Users/BNL28/Code/DataPerformance/bug_report.py", line 33, in <module>
    comparative_results()
  File "/Users/BNL28/Code/DataPerformance/bug_report.py", line 26, in comparative_results
    ax = draw_plot(ax, df1, 'k')
  File "/Users/BNL28/Code/DataPerformance/bug_report.py", line 22, in draw_plot
    ax = data.boxplot(column=['x'], by=['z'], showfliers=False, ax=ax, color=colors)
  File "/Users/BNL28/anaconda3/lib/python3.6/site-packages/pandas/plotting/_core.py", line 2254, in boxplot_frame
    return_type=return_type, **kwds)
  File "/Users/BNL28/anaconda3/lib/python3.6/site-packages/pandas/plotting/_core.py", line 2223, in boxplot
    return_type=return_type)
  File "/Users/BNL28/anaconda3/lib/python3.6/site-packages/pandas/plotting/_core.py", line 2683, in _grouped_plot_by_column
    re_plotf = plotf(keys, values, ax, **kwargs)
  File "/Users/BNL28/anaconda3/lib/python3.6/site-packages/pandas/plotting/_core.py", line 2191, in plot_group
    bp = ax.boxplot(values, **kwds)
  File "/Users/BNL28/anaconda3/lib/python3.6/site-packages/matplotlib/__init__.py", line 1810, in inner
    return func(ax, *args, **kwargs)
TypeError: boxplot() got an unexpected keyword argument 'color'

Process finished with exit code 1 
```

**Expected outcome**

Expect two sets of box plots, one coloured black, and one coloured red. Code runs ok with no color keyword, but the boxes are indistinguishable without colour control.

**Environment**

  * Operating system: OSX
  * Matplotlib version: 3.0.2
  * Matplotlib backend (`print(matplotlib.get_backend())`):
  * Python version: Python 3.6.8 |Anaconda, Inc.| (default, Dec 29 2018, 19:04:46) 
  * Pandas version 0.24.2

