function setColor(element, color)
{
element.style.backgroundColor = color;
}
// where el is the concerned element
var el = document.getElementById('elementId');
setColor(el, 'green');
document.getElementById("ID").style.background = "colorName"; //JS ID
document.getElementsByClassName("ClassName")[0].style.background = "colorName"; //JS Class
Jquery:
$('#ID/.className').css("background","colorName") // One style
$('#ID/.className').css({"background":"colorName","color":"colorname"}); //Multiple style
Imagine you would like to apply CSS styles to more than one element, for example, make the background color of all elements with the class name boxlightgreen. Then you can: