I would like to define the border color of each node according to a 2nd category... is it possible?
Ex: node color = actor type (association, company...)
node border color = position of the actor about a specific subject (for, against...)
In the new version, you can override node renderers to make them plot a border. Since the colors are only used in the node renderer, you can just make them consider a borderColor attribute as well.
See how to develop your own node renderers in this wiki page.