gistfile1.js

    var nav = {
        id: 'nav#top-menu',
        anchor: 'nav#top-menu li',
        content: '#content',
        shadow: 'nav-box-shadow',
        boxShadow: function() {
            // Set box shadow at bottom of nav menu on window scroll.
            if ($(window).scrollTop() === 0) {
                $(nav.id).removeClass(nav.shadow);
            } else {
                $(nav.id).addClass(nav.shadow);
            }
        },
        scrollOpacity: function() {
            // Change the opacity of the top nav menu on window scroll.
            var a = parseInt($(nav.content).css('padding-top')),
                b = $(window).scrollTop(),
                c = b / a;

            // Constrain opacity between 0 and 1.
            c = (c < 0) ? 0 : c;
            c = (c > 1) ? 1 : c;

            // console.log(c);

            $(nav.id).css('background-color', 'rgba(255,255,255,' + c + ')');
        },
        invertTextColour: function() {
            /* Graduallly invert the colour of the anchor element as the page
             * scrolls, using RGB values. */

            var limits = {
                upper: 255,
                lower: 88
            };

            var color = limits.upper, 
                difference = limits.upper - limits.lower,
                contentTop = parseInt($(nav.content).css('padding-top')),
                windowPos = $(window).scrollTop(),
                percentage = windowPos / contentTop;

            percentage = (percentage < 0) ? 0 : percentage;
            percentage = (percentage > 1) ? 1 : percentage;

            color = Math.floor(limits.upper - (difference * percentage));
            color = (color > limits.upper) ? limits.upper : color;
            color = (color < limits.lower) ? limits.lower : color;

            $(nav.anchor).css('color', 'rgba(' + color + ',' + color + ',' + color + ', 1)');
        }