I've also tried highlighting black by varying the glossiness of the paint.  GW Chaos Black has a bit of a sheen to it, which can be accentuated by adding a small amount of either gloss varnish or future floor wax.  Some other blacks, such as the Adikolor Despair Black, are completely flat, so you can work from flat black shadows up to semi-gloss highlights (maybe even full gloss on black metal or polished leather).  I've only tried this on a couple of models, but it seems to work, and avoids the problem of making everything look grey.
I guess the same principal would work with other colours, although I've never tried.  It might be an interesting way to add texture to fabrics such as banners and horse-barding.