You can use any unoccupied Site from your territory for Site Access regardless of brigade color(s). A Site provides you access to other Sites of the same brigade.
I.e.: you have an unoccupied red Site and your opponent has an occupied red Site. You can use your red Site to give your Hero(es) access to the Soul in the opponent's red Site. Simply place the Site in battle as normal and that gives your Hero(es) access.
