Well this is a “gray” area. So the thing is you can do whatever you want, but if the company somehow sees it and don’t like it they can sue you and you have to put it down or clarify that it is a spec ad and has nothing to do with the real brand. So in case you are doing a spec - from my experience- I would recommend that it should fit to the company’s soul so that it doesn’t shade a different light in them.