ENH: accept a dictionary in plot colors
This is a resurrection of #28659. Have kept the original commit history, just rebased and added docstrings.

- [x] closes #8193
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
