If Sensei is absent on any given day, the highest belt present leads the warm up and instructs.
This was the same at our school.
When lining up the colored belts were lined up side by side, row by row facing the Instructor. The black belts were on the left (if looking from the Instructors point of view). They were lined up one behind the other starting with the highest Dan. Age also came into play regardless of who had their belt first when belts were held by the same people. Also it was mandatory that each helped in the class and lead some classes. The Red/Black* belt, it was mandatory prior to testing that they also had led some classes.
The same within our dojo. *Brown/Black belt.
When my instructor was contracted on the other side of the country and couldn't always be on time for class, he asked me - a brown belt at that time - to instruct or do the warm-ups.
@ Duane, are you training in a Okinawan karate (a guess: Shorin) school?
I have read about those stripes somewhere, but am not sure where.