That would first require that every monochrome post also gets that tag. Currently only a fraction of them do, though.
I think the color tag can be useful as tags indication an exception, i.e. a color version of something usually monochrome. For instance to tag the few color pics by an artist who usually works in monochrome (like Shichimenchou).
If every pic that uses color gets the color tag it would indeed be completely useless. Now getting people to use them properly might be harder...