You don't need AJAX for this, just some plain JavaScript setting the background-color property of the body element, like this:

document.body.style.backgroundColor = "#AA0000";

If you want to do it as if it was initiated by the server, you would have to poll the server and then change the color accordingly.

I agree with the previous poster that changing the color by className is a prettier approach. My argument however is that a className can be regarded as a definition of "why you want the background to be this or that color."

For instance, making it red is not just because you want it red, but because you'd want to inform users of an error. As such, setting the className AnErrorHasOccured on the body would be my preferred implementation.

In css

body.AnErrorHasOccured
{
  background: #f00;
}

In JavaScript:

document.body.className = "AnErrorHasOccured";

This leaves you the options of styling more elements according to this className. And as such, by setting a className you kind of give the page a certain state.

AJAX is getting data from the server using Javascript and XML in an asynchronous fashion. Unless you want to download the colour code from the server, that's not what you're really aiming for!

But otherwise you can set the CSS background with Javascript. If you're using a framework like jQuery, it'll be something like this:

$('body').css('background', '#ccc');

Otherwise, this should work:

document.body.style.background = "#ccc";

You can do it in following ways STEP 1

   var imageUrl= "URL OF THE IMAGE HERE";
   var BackgroundColor="RED"; // what ever color you want

For changing background of BODY

document.body.style.backgroundImage=imageUrl  //changing bg image
document.body.style.backgroundColor=BackgroundColor //changing bg color

To change an element with ID

document.getElementById("ElementId").style.backgroundImage=imageUrl
document.getElementById("ElementId").style.backgroundColor=BackgroundColor 

for elements with same class

   var elements = document.getElementsByClassName("ClassName")
        for (var i = 0; i < elements.length; i++) {
            elements[i].style.background=imageUrl;
        }

I wouldn't really class this as "AJAX". Anyway, something like following should do the trick:

document.body.style.backgroundColor = 'pink';

Css approach:

If you want to see continuous colour, use this code:

body{
    background-color:black;
    animation: image 10s infinite alternate;
    animation:image 10s infinite alternate;
    animation:image 10s infinite alternate;
}

@keyframes image{
    0%{
background-color:blue;
}
25%/{
    background-color:red;
}
50%{
    background-color:green;
}
75%{

    background-color:pink;
}
100%{
    background-color:yellow;
    }
  }  

If you want to see it faster or slower, change 10 second to 5 second etc.

Here are 2 ways to Change Background Color Using Javascript

1. To change background color with javascript you can apply style.background or style.backgroundColor on the element you want to change background for.

   The below example changes the background color of the body when you click an element using style.background property.

function pink(){ document.body.style.background = "pink"; }
function sky(){ document.body.style.background = "skyblue"; }

<p onclick="pink()" style="padding:10px;background:pink">Pink</p>
<p onclick="sky()" style="padding:10px;background:skyblue">Sky</p>

2. Another approach could be by adding a CSS class using javascript that gives a background to the element it is attached.

   You can see more ways to add class using javascript but here you can simply use classList.add() property.

       function addClass(yourClass){
           document.body.classList.remove("sky", "pink");
           document.body.classList.add(yourClass);
       }

       .pink {
           background: pink;
       }
   
       .sky {
           background: skyblue;
       }

       <p onclick="addClass('pink')" style="padding:10px;background:pink">Pink</p>
       <p onclick="addClass('sky')" style="padding:10px;background:skyblue">Sky</p>

You can change background of a page by simply using:

document.body.style.background = #000000; //I used black as color code

However the below script will change the background of the page after every 3 seconds using setTimeout() function:

$(function() {
            var colors = ["#0099cc","#c0c0c0","#587b2e","#990000","#000000","#1C8200","#987baa","#981890","#AA8971","#1987FC","#99081E"];

            setInterval(function() { 
                var bodybgarrayno = Math.floor(Math.random() * colors.length);
                var selectedcolor = colors[bodybgarrayno];
                $("body").css("background",selectedcolor);
            }, 3000);
        })

READ MORE

DEMO

I would prefer to see the use of a css class here. It avoids having hard to read colors / hex codes in javascript.

document.body.className = className;

if you wish to use a button or some other event, just use this in JS:

document.querySelector("button").addEventListener("click", function() {
document.body.style.backgroundColor = "red";
});

This will change the background color according to the choice of user selected from the drop-down menu:

function changeBG() {
  var selectedBGColor = document.getElementById("bgchoice").value;
  document.body.style.backgroundColor = selectedBGColor;
}

<select id="bgchoice" onchange="changeBG()">
    <option></option>
    <option value="red">Red</option>
    <option value="ivory">Ivory</option>
    <option value="pink">Pink</option>
</select>

Add this script element to your body element, changing the color as desired:

<body>
  <p>Hello, World!</p>
  <script type="text/javascript">
     document.body.style.backgroundColor = "#ff0000";  // red
  </script>
</body>

<!DOCTYPE html>
<html>
<body>
<select name="" id="select" onClick="hello();">
    <option>Select</option>
    <option style="background-color: #CD5C5C;">#CD5C5C</option>
    <option style="background-color: #F08080;">#F08080</option>
    <option style="background-color: #FA8072;">#FA8072</option>
    <option style="background-color: #E9967A;">#E9967A</option>
    <option style="background-color: #FFA07A;">#FFA07A</option>
</select>
<script>
function hello(){
let d = document.getElementById("select");
let text = d.options[d.selectedIndex].value;
document.body.style.backgroundColor=text;
}
</script>
</body>
</html>

Alternatively, if you wish to specify the background color value in rgb notation then try

document.getElementById("yourid").style.backgroundColor = `rgb(${a}, ${b}, ${c})`;

where a,b,c are the color values

Example:

document.getElementById("yourid").style.backgroundColor = 'rgb(224,224,224)';

so you can done very easily by just calling a function like

 function changeBg()
    {
      document.body.style.color.backgroundColor="#ffffff";

    }

But you would want to configure the body color before the <body> element exists. That way it has the right color from the get go.

<script>
    var myColor = "#AAAAAA";
    document.write('\
        <style>\
            body{\
                background-color: '+myColor+';\
            }\
        </style>\
    ');
</script>

This you can put in the <head> of the document or in your js file.

Here is a nice color to play with.

var myColor = '#'+(Math.random()*0xFFFFFF<<0).toString(16);

I would suggest the following code:

<div id="example" onClick="colorize()">Click on this text to change the
background color</div>
<script type='text/javascript'>
function colorize() {
var element = document.getElementById("example");
element.style.backgroundColor='#800';
element.style.color='white';
element.style.textAlign='center';
}
</script>

You can change background of a page by simply using:

function changeBodyBg(color){
    document.body.style.background = color;
}

Read more @ Changing the Background Color

It can be achieved using

document.body.style.color.backgroundColor = "#000000";