$(function() {

    var highlighted = null,         // Keep track of previously selected
        $menuItems  = $('#menu a'); // Grab all menu items once

    // Which items at which points
    var breakpoints = {
        '#menu-item-25': 590,
        '#menu-item-26': 1380,
        '#menu-item-22': 2545,
        '#menu-item-23': 2969,
        '#menu-item-24': 99999
    };

    $(window).scroll(function() {

            var scroll = $(window).scrollTop(),
            selected = '#menu-item-24';  // default

        // Loop through breakpoints to find the current one that should be selected
        for (id in breakpoints) {
            if (scroll <= breakpoints[id]) {
                selected = id;
                break;
            }
        }

        // If the right one is already highlighted, do nothing
        if (highlighted == id) {
            return;
        }

        // Remove all
        $menuItems.removeClass('blue');
        // Add it to the one you need
        $(id).addClass('blue');

    });
});