// global variable that keeps track of if we are red (true) or green (false)
let strandRed = true;

function update(lights) {

  // Loop over all the lights 
  for (let i=0; i<100; i++) {
    if (strandRed) {
      // Set red
      lights[i].rgb(1, 0, 0);
    } else {
      // Set green
      lights[i].rgb(0, 1, 0);
    }
  }

  strandRed = !strandRed;

  // Wait 500 ms until we get called again.
  return 500;
}