The config file is stored in a system dependent location.
On linux this should be something like:
On Windows this is in the registry:
Default values on nodes can be changed as explained in the doc.
The colour of the pyplug is currently not possible to edit from the GUI at export time. You would have at the end of the createInstance function to write the following:
# rgb must be between 0-1
group.setColor(r, g, b)