Make color validation more forgiving
The current version throws a false positive if the style string contains `s` or `o`, which are valid marker styles and not colors.  For example:

```
style='s', color='C3'
```

should be legal, but currently throws an error.

My suggestion is to check for only the letters that are color codes.

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