TST: Add hex color strings test
It's already resolved. I added hex string color test.

closes #10299

Thanks! This looks to be fixed by #9894. 

@TomAugspurger Please merge if it looks OK.

```
df = pd.DataFrame(np.random.randn(5, 5))
custom_colors = ['#FF0000', '#0000FF', '#FFFF00', '#000000', '#FFFFFF']
df.plot(color=custom_colors)
```

![2015-10-12 17 00 39](https://cloud.githubusercontent.com/assets/1696302/10423585/d4ed2e32-7102-11e5-9b0d-29901d8f4805.png)
 @hironow pls add a whats new note (bug fxies)
 Sorry, unexpectedly closed.
I tried to 'git rebase' (and replace this PR's repo) to resolve conflicts, but I deleted this by mistake. 
So, what should I do about this PR?
 Does Github show a button for you to restore the branch?
 @TomAugspurger Thanks for your comment. I restore this branch.
I've created a branch of duplicate name in my own forked repo.
 Thanks for updates. Can you squash to the single commit? You can do it by:
1. Open your local pandas repository
2. `git checkout master` to switch to master
3. `git remote add upstream https://github.com/pydata/pandas.git` to register original url
4. `git pull upsream master` to update your master
5. `git checkout pyconjp-2015-sprint`
6. `git rebase -i master` to apply the master's update to PR branch

Pls feel free to ask me if you have any question.
 @sinhrks Thanks for your comment and list of git commands.
I probably was able to squash this branch.
 @hironow Thanks!
