Use nth-child(6n) and nth-child(7n)
const container = document.getElementById('container');
for (let i = 1; i <= 300; i++) {
  const elm = document.createElement('div');
  elm.innerText = i;
  container.append(elm);
}#container div:nth-child(6n), #container div:nth-child(7n) {
  color: red;
}<div id="container"></div>
solved Styling loop element in for in javascript