Coldplay stage was somewhat shaped like a guitar.  At the entrance gates the ticket takers gave everyone a wristband that you were asked to wear.  When Coldplay took the stage, the lights went out and everyone's wristband lit up with a dozen LEDs sewn into the fabric.  There were about 7 or 8 different colors of wristbands.  Periodically throughout their performance, the wristbands would light up and blink to the music.  It was awesome!