<button>Change color</button>

button {
  margin: 10px;
}

const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() * (number + 1));
}

btn.addEventListener("click", () => {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
});