Two standard sized red pencils. People use red pencils for a variety of purposes, including:
Corrections
Red pencils are often used to correct written copy. 
Sketching
Artists often use red pencils for preliminary sketches because they allow for a looser, more non-committal style. The red lines can be easily filtered out when the artwork is scanned or photographed for digital use. 
Precision erasing
The red end of a pencil eraser can be used for precision erasing on light paper. 
Comic art
Comic artists use colored pencils to isolate sketch lines so they can be removed after inking.