<section class="flex-sec">
  <button>BTN</button>
  <div>1</div>
  <div>2</div>
  <div>3</div>
  <div>4</div>
  <div>5</div>
</section>

section {
  display: flex;
  button {
    width: 50px;
    height: 50px;
    border: solid 4px black;
    background: yellow;
  }
}
div {
  width: 200px;
  height: 200px;
  background: #00ff0b;
  margin: 10px;
}

.b1 {
  background: #fc0;
}

let cards = document.querySelectorAll(".flex-sec > div");
let button = document.querySelector(".flex-sec>button");
let firstClick = true;

button.addEventListener("click", switchColor);

function switchColor() {
  for (let i = 0; i < cards.length; i++) {
    if (firstClick) {
      if (i % 2 == 0) {
        cards[i].classList.add("b1");
      }
    } else {
      cards[i].classList.toggle("b1");
    }
  }
  firstClick = false;
}