added check for 2 elements

Author: jamesmkrieger

prody/dynamics/plotting.py

@@ -261,7 +261,9 @@ def showProjection(ensemble, modes, *args, **kwargs):
     c = kwargs.pop('c', 'blue')
     colors = kwargs.pop('color', c)
 
-    if isListLike(colors) and not len(colors) in [3, 4]:
+    if isListLike(colors) and len(colors) == 2:
+        raise ValueError('each entry of color should have 1, 3 or 4 values not 2')
+    elif isListLike(colors) and not len(colors) in [3, 4]:
         colors = list(colors)
     elif isinstance(colors, str) or colors is None or isListLike(colors):
         colors = [colors] * num