BUG: Extra color introduced with plot.
User reported in StackOverflow: https://stackoverflow.com/questions/49887076/matplotlib-error-while-coloring-barplot-with-list#49887076
```python
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame(np.abs(np.random.randn(48, 4)), columns=list("ABCD"))

color_list = plt.cm.gnuplot(np.linspace(0,1,16))

df.A.plot.bar( figsize=(16,7), color=color_list)
plt.axvline(15.5, c="black", lw=1)
plt.axvline(31.5, c="black", lw=1)
plt.plot()
```
#### Problem description
This bug happens when single color is needed for plotting but an array is provided. 
In the end of `_get_standard_colors` function in plotting/_style.py. An extra first item will be added to the color array which caused the circular misbehaved in such case.
![image](https://user-images.githubusercontent.com/659168/38904144-94530962-425d-11e8-82c1-2b5767caba2c.png)

#### Expected Output
The bar 17 should have the same color as the bar 1.
#### 