Below are types of statements you could make in order to more thoroughly explain the design choices you've made in your Documents.
I chose the script font Old English because I wanted to convey the ethos of my subject, the Carolina Renaissance festival, and the font gives my document that "olde" time look that characterizes contemporary assumtpions about the time period, which is obvious based on fonts used on period pieces in films, TV shows, books, and festivals related to the time period .
My document is green because that gives off an environmental ethos, which supports my document's goal--cleaning up the environment. The culture associates green with environmental concerns in talk of "green jobs," "going green," "green companies," etc. In fact, even the environmentally oriented political party in the United States is called the Green Party.
I chose the font Comic Sans MS because I wanted to make an appeal to pathos because my design is for kids, and the font has a vibrant, goofy, childish feeling. It looks like the typeface that would be used in K-6 classrooms.
I used red for pathos in my document to create a sense of danger. My document warns readers to take precautions when cleaning their firearms, and the color red is associated with danger in our/mine/this culture.
***Notice how the above descriptions state a reason and then explain, specifically, why the statement is valid. Basically, the second part of each is proof of the preceeding claim. And you must have some kind of proof when discussing why you think you design choice conveys what you claim it conveys.