Added a check on color argument that will issue a warning.

Not sure if need to raise TypeError or issue a UserWarning if a list with more than two elements is passed.

Author: jmarrec

pandas/formats/style.py

@@ -19,6 +19,8 @@
 
 from pandas.types.common import is_float, is_string_like, is_list_like
 
+import warnings
+
 import numpy as np
 import pandas as pd
 from pandas.compat import range
@@ -977,7 +979,7 @@ def bar(self, subset=None, align='left', axis=0,
             If a str is passed, the color is the same for both
             negative and positive numbers. If 2-tuple/list is used, the
             first element is the color_negative and the second is the
-            color_positive (eg: ['d65f5f', '5fba7d'])
+            color_positive (eg: ['#d65f5f', '#5fba7d'])
         width: float
             A number between 0 or 100. The largest value will cover ``width``
             percent of the cell's width
@@ -998,6 +1000,15 @@ def bar(self, subset=None, align='left', axis=0,
 
         if not(is_list_like(color)):
             color = [color, color]
+        elif len(color) == 1:
+            color = [color[0], color[0]]
+        elif len(color) > 2:
+            msg = ("Must pass `color` as string or a list-like"
+                   " of length 2 [ `color_negative`, `color_positive`] "
+                   "(eg: color=['#d65f5f', '#5fba7d'])\n"
+                   "Only the first two list elements will be used here.")
+            warnings.warn(msg, UserWarning)
+            color = [color[0], color[1]]
 
         if align == 'left':
             self.apply(self._bar_left, subset=subset, axis=axis, color=color,