I tried the example with several colormaps (have you checked out
cubehelix which nicely resembles the grey scale visual intensity
distribution in color) and I definitely agree that it would be good
for matplotlib to switch to a more sensible default color map. My
personal vote goes to coolwarm which has well defined behaviour and is
suitable (e.g. looks nice) for a wide range of applications.
P.S. Although cubehelix also has well defined behaviour it is less
optimal as a default since it does not look nice in all use cases (but
is very good in some, particularly for cases where the percieved
intensity distribution needs to be the same when viewed on screen and
printed in black and white).