content.js

chrome.runtime.onMessage.addListener(function(color) {
  if (color === 'changeToYellow') {
    document.body.style.color = 'yellow';
  }
});

popup.html

<div>
  <button id="button1">First</button>
  <button id="button2">Second</button>
</div>

popup.js

document.getElementById('button1').addEventListener('click', function(){
  // make the page yellow
  alert('button 1 was clikced');
  chrome.tabs.query({active...}, function(tabs) {
    chrome.tabs.sendMessage(tabs[0].id, 'changeToYellow');
    
  })
})

document.getElementById('button2').addEventListener('click', function(){
  alert('button 2 was clikced');
  // make the page blue
})