[color[#ff0000]tagging[]]

Note that, in theory, the first filter step color[#ff0000] could find other tiddlers that aren't tags and just so happen to have a color field set to #ff0000. However, this still gives the correct output because any tiddler that tags other tiddlers is by definition a tag, and tagging[] will ignore any input tiddler that doesn't tag at least one other tiddler.