BUG:  Fix a bug in plotting when using color array. #20726
- [x] closes #20726 
- [x] tests added / passed
- [x] passes `git diff master --name-only -- "*.py" | flake8`
- [x] whatsnew entry
you would need a test for this & a whatsnew entry. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20727?src=pr&el=h1) Report
> Merging [#20727](https://codecov.io/gh/pandas-dev/pandas/pull/20727?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/100ffffa0ace51baac7045ca51fecf228e54bc5b?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20727/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20727?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #20727   +/-   ##
=======================================
  Coverage   31.89%   31.89%           
=======================================
  Files         166      166           
  Lines       52421    52421           
=======================================
  Hits        16722    16722           
  Misses      35699    35699
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.29% <0%> (ø)` | :arrow_up: |
| #single | `31.89% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20727?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_style.py](https://codecov.io/gh/pandas-dev/pandas/pull/20727/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19zdHlsZS5weQ==) | `23.65% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20727?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20727?src=pr&el=footer). Last update [100ffff...892399a](https://codecov.io/gh/pandas-dev/pandas/pull/20727?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Can you merge master for this? Hello @wuhaochen! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/plotting/_style.py`](https://github.com/wuhaochen/pandas/blob/0aa529f61e6996c51d37d0eb28ed8342259aa83f/pandas/plotting/_style.py) !

 - There are no PEP8 issues in the file [`pandas/tests/plotting/test_misc.py`](https://github.com/wuhaochen/pandas/blob/0aa529f61e6996c51d37d0eb28ed8342259aa83f/pandas/tests/plotting/test_misc.py) !

 @jreback is anything else needed here, or can this be merged? looks fine. if someone wants to push the changes @mroeschke suggested would be great. @WillAyd any comments. thanks @wuhaochen 