I think it's important to remember that all this is murky from a legal point of view. Anyone who tells ou differently has probably never beein involved in a lawsuit.
That's why the BEST solution is for all parties to try to treat each other with respect. The ability to enforce ones intellectual property rights is depressingly difficult - believe me I know from experience with site flagrantly pirating our stuff or sites using our images and even derivatives of our logo.
So it's not a cut and dry thing. For instance, showing a screenshot of a WindowBlinds skin colorized as a different color would not constitute any sort of violation in my opinion. WindowBlinds is made to colorize skins on the fly and any skin author making skins for it knows this.
Moreover, the example I gave for The Skins Factory has to do with them making a commercial skin which is very different. When financial damage comes into play, things change.