<div class="box">
  <div class="item red">red</div>
  <div class="item yellow">yellow</div>
  <div class="item blue">blue</div>
  </div>

const child = document.querySelectorAll('.item');
let box = document.querySelector('.box');

child.forEach(el => {
  el.addEventListener('click', () => {
    if(el.matches('.red')){
      box.style.background = "red";
    }
    if(el.matches('.yellow')){
      box.style.background = 'yellow';
    }
    if(el.matches('.blue')){
      box.style.background = 'blue';
    }
  })
});