BUG: DataFrame.plot raises ValueError when color name is specified by multiple characters 
Derived from #9894. Passing color name with multiple characters results in `ValueError`. Below is the bahavior of current master.

```
# OK
df = pd.DataFrame(np.random.randn(3, 3))
df[0].plot(color='green')
# single green line

# OK
df.plot(color=['green'])
# triple green lines

# NG
df.plot(color='green')
# ValueError: to_rgba: Invalid rgba arg "e"
# -> This should be triple green lines
```

If passed str can be parsed as both single color and color cycle, following error will be raised.
- _"'green' can be parsed as both single color and color cycle. Specify each color using a list like ['green'] or ['g', 'r', 'e', 'e', 'n']"_

Currently, there is no color name which can meet above condition (thus cannot tested).
- http://matplotlib.org/examples/color/named_colors.html
