improve docstring and add test fot to_rgb(<float>)

Even if the docstring was saing it, emphasis that to_rgb/to_rgba do not
accept float as input, but string representation of a float.

As one might be tempted to had float->gray conversion add failing test
in case the functionality is added.

Side fixes :
    change tuple([value]*3) to (value,)*3 for speed

In [18]: %timeit  tuple([0.4]*3)
1000000 loops, best of 3: 505 ns per loop

In [19]: %timeit  (0.4,)*3
10000000 loops, best of 3: 23.5 ns per loop

In [20]:  (0.4,)*3 == tuple([0.4]*3)
Out[20]: True

Closes matplotlibgh-2609

lib/matplotlib/colors.py

@@ -272,10 +272,13 @@ def to_rgb(self, arg):
             1) a letter from the set 'rgbcmykw'
             2) a hex color string, like '#00FFFF'
             3) a standard name, like 'aqua'
-            4) a float, like '0.4', indicating gray on a 0-1 scale
+            4) a string representation of a float, like '0.4',
+               indicating gray on a 0-1 scale
 
         if *arg* is *RGBA*, the *A* will simply be discarded.
         """
+        # Gray must be a string to distinguish 3-4 grays from RGB or RGBA.
+
         try:
             return self.cache[arg]
         except KeyError:
@@ -304,7 +307,7 @@ def to_rgb(self, arg):
                         if fl < 0 or fl > 1:
                             raise ValueError(
                                 'gray (string) must be in range 0-1')
-                        color = tuple([fl] * 3)
+                        color = (fl,)*3
             elif cbook.iterable(arg):
                 if len(arg) > 4 or len(arg) < 3:
                     raise ValueError(

lib/matplotlib/tests/test_colors.py

@@ -192,6 +192,19 @@ def test_autoscale_masked():
     plt.draw()
 
 
+def test_colors_no_float():
+    # Gray must be a string to distinguish 3-4 grays from RGB or RGBA.
+
+    def gray_from_float_rgb():
+        return mcolors.colorConverter.to_rgb(0.4)
+
+    def gray_from_float_rgba():
+        return mcolors.colorConverter.to_rgba(0.4)
+
+    assert_raises(ValueError, gray_from_float_rgb)
+    assert_raises(ValueError, gray_from_float_rgba)
+
+
 if __name__ == '__main__':
     import nose
     nose.runmodule(argv=['-s', '--with-doctest'], exit=False)