BUG: .plot modifing `colors` input
Closes https://github.com/pydata/pandas/issues/12039

Just added a defensive copy in `_get_standard_colors` on list-likes.

Travis had a (seemingly very unrelated) failure on one of the [allowed to fail builds](https://travis-ci.org/pydata/pandas/jobs/102507481). If anyone wants to merge this tonight feel free, otherwise I'll restart the travis build tomorrow and assume it was a random failure.
