​​What are those colourful Balls attached to Power lines?

They’re called visibility marker balls or marker balls. You’ll often find them near mountain passes, in the deep valley areas, near major freeway crossings and around airports. They weight about 17 pounds each. 
The marker balls are placed on power lines to make the conductor crossings visible to aircraft. Helicopters and small aircraft often fly low in mountain passes or freeways and usually fly low while approaching an airport.
When you are flying, the conductors and skyline become almost invisible against the terrain — especially the skyline wires.
Many of the marker balls are required by the Federal Aviation Administration, like the ones near airports.