df.plot bars with different colors depending on values
I found a strange behavior with pandas.plot colors my version of pandas is

```python
import pandas as pd
pd.__version__
'0.20.3'
```
#### Problem description

Before when i wanted to assign different colors to bars depending on value i could simply do

```python
n=10
df = pd.DataFrame({"a":np.arange(1,n)})
df.plot(kind='bar', color=np.where(df["a"]>2, 'g', 'r'))
```
Now not only it accepts only a `tuple` instead of an array as color but it breaks whenever the `tuple` is longer than `5`
```python
n=5
df = pd.DataFrame({"a":np.arange(1,n)})
colors = tuple(np.where(df["a"]>2, 'g', 'r'))
df.plot(kind='bar', color= colors )
```

```python
n=6
df = pd.DataFrame({"a":np.arange(1,n)})
colors = tuple(np.where(df["a"]>2, 'g', 'r'))
df.plot(kind='bar', color= colors )
```
