Rubricate is a verb that means to color red. 

The Latin word rubrica (ROO brick uh) means ‘red.’ By the Late 16th century, this word had been imported to English and turned into a verb which meant to turn something red.’ Here’s an example:

The author decided to rubricate the letters that indicated the location of the main character. By having this information in red, there was no chance the reader could confuse his location with other locations.