levels = np.linspace(1,3,11)[1:-1]
cmap = copy.deepcopy(plt.cm.Spectral)
cmap.set_under('silver')
cmap.set_over('w')
fig, ax = plt.subplots()
cs = ax.contourf(v, levels, cmap=cmap, extend='both')
fig.colorbar(cs, aspect=40, pad=0.1, orientation='horizontal',
extendfrac=1./(len(levels)-1))
fig, ax = plt.subplots()
cs = ax.contourf(v, levels, cmap=copy.deepcopy(plt.cm.Spectral), extend='both')
cs.cmap.set_under('silver')
cs.cmap.set_over('w')
cs.changed()
fig.colorbar(cs, aspect=40, pad=0.1, orientation='horizontal',
extendfrac=1./(len(levels)-1))