What about this one?

final static short ELAPSE = 3000;
int timer;
boolean changeBG;

void setup() {
  size(400, 400);
}

void draw() {
  if (millis() > timer) {
    timer = millis() + ELAPSE;
    changeBG = !changeBG;
  }

  background(changeBG? #FFFF00 : #00FFFF);
}

As you know, millis() returns how much time transpired since program has started. And you can't reset it!
Strategy I used was to set a timer ahead in the future. That is, ELAPSE milliseconds ahead of current millis().

At each draw() call, a check is made to know whether current millis() has reached timer value.
Of course, this is gonna take exactly ELAPSE milliseconds to become true!

Once it triggers, do it all over again. Plus, switch boolean flag changeBG state.
So the background() color keeps shifting one color to the other!