Conversation so far:
what are the balls in power lines for
The marker balls are placed on power lines to make the conductor crossings visible to aircraft.
when were the balls in power lines introduced
The marker balls first gained popularity in the early 1970s, when Arkansas’s then-governor Winthrop Rockefeller decided something should be done to make them more obvious to pilots.

Question: what is the most common color
Self-contained rewrite: what is the most common color for the balls in power lines