A trademark is a device that is like your signature. You have to use it for it to be a trademark. You can't pre-emptively trademark things and sue people for using them, when you had no use for the mark.
The original purpose was, literally, to "sign" the things you made in your trade, so that people knew what to expect from your quality. Over time, it became brand names and slogans as well. You can also trademark colors, etc., so long as they are not functional attributes of the item. The best known example is John Deere Green. There is no reason why a tractor has to be painted green, so the decision to brand all their equipment green is their trademark, and they protect it. On the other hand, if you were to try to trademark day-glo yellow for fire trucks, one could argue that the bright color is a functional aspect of the device and therefore not a trademark.
What you might be looking for is something akin to a nondisclosure agreement. You are going to present slogan ideas for universities. You want them to pay a royalty for that. So you say, sign this NDA, I'll tell you the slogan, if you want it you pay me a royalty, and you can trademark it. If you don't want it, you have agreed not to use it.
Another thing you can do is make a business that has a trademark on a part of a slogan that you then specialize. You would be a sort of design firm for universities, offering to sell them designs with your slogan on them. This would be like the "Co-ed Naked" gear.
http://www.uspto.gov/trademarks/basics/index.jsp(This is not legal advice)