Identify all unique boundary colors, after all boundary attributes and seeds are known.
This has to be done separately from reading the attributes, due to the interdependence of the boundaries and seed objects. It has to be ensured that all seed objects are known at this point.
The bc_colors will correspond to the unique color name list, or are 0 if the bc color does not exist in that list. 'none' bc colors are marked negatively as they are to be applied to all seed colors.