How do I change the background color with JavaScript?

Modify the JavaScript property document.body.style.background.

For example:

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

window.addEventListener("load",function() { changeBackground('red') });

Note: this does depend a bit on how your page is put together, for example if you’re using a DIV container with a different background colour you will need to modify the background colour of that instead of the document body.