Names can refer to both universals and particulars, but not in the same way. The difference between a personal name such as Stephen and a universal such as red is that names of particulars never have content insofar as they name particulars. They are a pure reference. Names of universals, on the other hand, always have content.
Reference and content are two elements of speech. Every sentence will contain both of them. Consider the sentence red is a color. It says of red, that it is a color. What it is about is the reference. A sentence that said Mike is not here refers to Mike. A sentence that said I like explosions is about me. What a sentence is about is its reference. Content is that part of the sentence that is not reference. So in the sentence red is a color color; and red; are both content, and joining them together is also content. Similarly, in the sentence Mike is not here, Mike and here are separated. Lastly, in the sentence I like explosions, explosions like and the joining of these with myself are all content.
Another way to view the distinction is that content is what something means while reference is what something is about. Redness refers to all red things and means red. I like explosions refers to myself and means that I like explosions. These intuitive distinctions are necessary when it comes to distinguishing between particulars and universals.
Anything that names a particular does so by reference alone. For example, Mike names a particular person and is the verbal equivalent of pointing. The reason for this is that no feature of a particular makes that particular unique. It is possible that a different particular could have that feature. Anyone could be born with an identical twin who had the same genetic code and the same parents, and we could have had a completely different history. So I cannot be named by picking out any feature currently true of me. And what is true of me is also true of every other particular.