Yes, merge it into faces and make it a synonym. Other tags like font-lock or text properties might be worth adding to some of these questions, but they have more distinct meanings and that's a case by case thing. The tag color as a whole should be a synonym of faces.
Out of all current uses, the only one where color suits better than faces is: Are there color pickers for Emacs? . And even here faces would be an acceptable tag (though we can manually find a more appropriate one for this).
As a second point, what would be the purpose of a color tag? A color, in elisp, is nothing but a face property. Having a color tag is like having a weight tag or an underline tag. It's too small to warrant a tag (as exemplified by the total of 1 use so far where it works better than faces).