Scatter plot colour incorrect for 3 scatter points.
Based on the Stack Overflow question [here](http://stackoverflow.com/q/29253027/3005188) I believe I've found a bug for when a scatter plot only has three points. The full details are in the question but I'll sketch out the details here.

The code (below) will create a scatter plot where each of the points has a different colour, rather than a single consistent colour (as is expected).

It happens particularly when `seaborn` is used because, by default, `seaborn` will modify the `rcParams['patch.facecolor']` to be an RGB tuple. If you don't use `seaborn` then this value defaults to a string `'b'` which indicates the colour blue. Of course, this bug would arise whenever a user may use a custom `patch.facecolor` that happens to be an RGB tuple, so it isn't specifically restricted to `seaborn`.

The issue arises because when a sequence of numbers is passed for the `c` argument with matplotlib, matplotlib will try to map it to the current colormap. When there are only three scatter points the sequence lengths match (i.e. `len(data) == len(colors)`) and so the numbers are mapped and the resulting colours used in the plot.
### Reproducing code

``` python

from pandas import DataFrame
import matplotlib.pyplot as plt
import seaborn as sns
import numpy as np

df = DataFrame({'x': np.random.uniform(0, 1, 3), 'y': np.random.uniform(0, 1, 3)})
df.plot(kind = 'scatter', x = 'x', y = 'y')
plt.show()
```

![Example plot](http://i.imgur.com/OJvQXsR.png)
### Versions

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.4.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.15.0
nose: 1.3.3
Cython: None
numpy: 1.9.0
scipy: 0.14.0
statsmodels: 0.5.0
IPython: 2.4.1
sphinx: 1.2.2
patsy: 0.3.0
dateutil: 2.2
pytz: 2014.7
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.4.2
openpyxl: 2.0.5
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
```
