<div id="testBlock">Lorem ipsum</div>
# <input type="text" onkeyup="changeColors()" id="input">

#testBlock {
  width: 128px;
  height: 64px;
  background: #000;
  color: #fff;
  margin-bottom: 16px;
  text-align: center;
}

function changeColors() {
  let input = document.getElementById("input");
  if (
    input.value == "fff" ||
    input.value == "ffffff" ||
    input.value == "FFF" ||
    input.value == "FFFFFF"
  ) {
    document.getElementById("testBlock").style.color = "#000";
  }
  if (input.value == "000" || input.value == "000000") {
    document.getElementById("testBlock").style.color = "#FFF";
  }
  document.getElementById("testBlock").style.background = `#${input.value}`;
}