every N frames

everyNframes.pde

color c = color(0);

void setup() {
  size(200, 200);
}

void draw() {
  background(c);

  // % is the remainder http://processing.org/reference/modulo.html
  // This will execute every 100 frames (0, 100, 200, 300, etc.)
  if (frameCount % 100 == 0) {
    c = color(random(255));
  }
}