How to change the color of a div every 40 seconds?

You can do this with jQuery simply, as follows:

$(document).ready(function() {
    var colors = ["#f9f9f9", "#049fce"];//Array com as cores, pode adicionar contas quiser
    var i = 0;
    setInterval(function(){
      $('.divclass').css('background', colors[i]);
      i = (i == (colors.length -1)) ? 0 : i+1;
    },40000);//40 Segundos em Milisegundos
});

Or with pure JavaScript:

var colors = ["#049fce", "#f9f9f9"];//Array com as cores, pode adicionar contas quiser
var i = 0;
setInterval(function(){
  document.getElementById('divid').style.background = colors[i];//A div deve ter um id, não funciona com class
  i = (i == (colors.length -1)) ? 0 : i+1;
},40000);//40 Segundos em Milisegundos

Select the div you want to change the color and use a setTimeout function with the number of seconds you need, in this case, 40000, use an if to make a counter always be the two indexes of an array with two colors, see:

const colorDiv = document.querySelector('.change-color');
let counter = 0;

const colorArr = ['red', 'yellow']

setInterval(
	()=> {
		counter++
		
		if (counter == colorArr.length) {
			counter = 0
		}
		
		colorDiv.style.background = colorArr[counter];
	}, 3000
)

.change-color {
	background: red;
	width: 300px;
	height: 300px;
}

<div class="change-color"></div>