This is a big issue. I suspect that the purists will consider color a distraction (see
http://forums.omnigroup.com/showthre...ighlight=color), but personally I find that color provides an extremely useful layer of metadata quite independent of the formal stuff. Some people seem to think in color whereas others just do not. The beauty of it if you do, is that whatever meaning you yourself associate with a particular colour is perceived more quickly and directly than by any other means.
Of course any implementation of color labels (something that Omni apps typically handle very well) would not be compulsory, so it need have no impact at all on the interface for those who choose to ignore it, but the the availability of color for me at least is a deal-maker as far as OF is concerned.