Try this :
var t = $('#about').offset().top - 100;
var t1 = $('#work').offset().top - 100;
$(document).scroll(function(){
if($(this).scrollTop() > t1) {
$('header').css({"background-color":"blue"});
} else if($(this).scrollTop() > t) {
$('header').css({"background-color":"green"});
} else {
$('header').css({"background-color":"#520833"});
}
});
And so on with var t2 = ...
.
Don't forget to put higher values on top of the if
's