Yes, merge it into [tag: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. 

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Out of all current uses, the only one where color suits better than faces is: https://emacs.stackexchange.com/questions/5582/are-there-color-pickers-for-emacs . And even here [tag: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 attribute. 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 [tag:faces]).