splash.js

// https://jsfiddle.net/v8Lxazhr/

function changeValue(isIncreasing) {
  const valueElement = document.getElementById('valueElement');
  let currentValue = parseInt(valueElement.innerText);

  if (isIncreasing) {
    currentValue++;
    valueElement.style.color = 'green';
  } else {
    currentValue--;
    valueElement.style.color = 'red';
  }

  valueElement.innerText = currentValue;

  setTimeout(() => {
    valueElement.style.color = '';
  }, 500);
}