On the left Michelle Lamb is adding interior lines to Holmes Library, while on the right Kristy Brucale Jach does the same for the First Presbyterian Church.
Notice the difference in these two photos between the areas where the line drawing has been added and the areas where it's still only the color. The dark lines really make the mural "pop." It also helps to mimic my drawing style.
It's hard work because of the precision required (fortunately both Michelle and Kristy have master-level technical skills) but it's also very gratifying to see the piece emerge right under your hand.