For anything really fine-grained, you're going to have to resort to keywords.

The "concensus method" I've seen among people who give talks and seminars and such has been to use star ratings as a sort of hybrid processsing status and quality indicator and colour codes as a broad "audience" indicator (reserving two colours for pano and HDR source sets).

Mind you, these tend to be systems used by specialist pros (wedding and event shooters in particular) who can easily create such "audience" categories (large print candidates, album candidates, vendor pics — gotta keep the caterers and florists happy if you want referrals —, web order pics for family and guests). For someone who is more generalist, the top-level categories become pretty broad, and features that only allow you a handful of options can only get you so far, so you have to pick a useful top-level category for your colours and use keywords for everything else.

It's a pretty standard database problem, really — you don't really know until v2.0 what your database schema should have looked like to begin with, and by then it's just too big a pain in the butt to migrate the data.