Trigger action when a user scrolls past a certain part of the page

TiggerOnScroll.js

$(window).scroll(function() {
    var y_scroll_pos = window.pageYOffset;
    var scroll_pos_test = 500;             
	// set to whatever you want it to be

    if(y_scroll_pos > scroll_pos_test) {
	   $("body").css("background-color","#000");
    }
	else
	{
		$("body").css("background-color","#FFF");
	}
});