Quick fork of jquery.brightness.js, but without the jQuery dependency.

brightness.js

function brightness(el) {
    var bgColor, rgba, y, parent;
    bgColor = el.style.backgroundColor;
    parent = el.parentNode;

    function reportParent() {
        return parent ? brightness(parent) : undefined;
    }

    if ((bgColor !== null) && bgColor.length) {
        rgba = bgColor.match(/^rgb(?:a)?\(([0-9]{1,3}),\s([0-9]{1,3}),\s([0-9]{1,3})(?:,\s)?([0-9]{1,3})?\)$/);
        if (rgba !== null) {
            if (rgba[4] === '0') {
                return reportParent();
            } else {
                y = 2.99 * rgba[1] + 5.87 * rgba[2] + 1.14 * rgba[3];
                if (y >= 1275) {
                    return 'light';
                } else {
                    return 'dark';
                }
            }
        }
    } else {
        return reportParent();
    }
}