Plotting methods modify color arrays in place
Ran into a weird issue where pandas plotting was changing values in color arrays I was passing to it.

Here is a simple example (Pandas 0.17.1; Python 2.7; OSX):

``` python
colors = sns.color_palette("Blues", 3)
print len(colors)
pd.DataFrame(np.random.rand(10, 2)).plot(color=colors)
print len(colors)
```

outputs:

``` python
3
5
```

I've seen various other kinds of changes, including sometimes changing the values but not the length of the color array. A particularly nasty one was where I had taken some actual data and passed it through a matplotlib norm and colormap, and the modifications made their way all the way back up to my data.

I assume this can easily be handled by copying the color array, but I'm not sure.

Sorry if this has been reported...I checked recent issues and didn't see anything, but there are a lot :)
