function blendColors() {
    var args = [].prototype.slice.call(arguments);
    var base = [0, 0, 0, 0];
    var mix;
    var added;
    while (added = args.shift()) {
        if (typeof added[3] === 'undefined') {
            added[3] = 1;
        }
        // check if both alpha channels exist.
        if (base[3] && added[3]) {
            mix = [0, 0, 0, 0];
            // alpha
            mix[3] = 1 - (1 - added[3]) * (1 - base[3]);
            // red
            mix[0] = Math.round((added[0] * added[3] / mix[3]) + (base[0] * base[3] * (1 - added[3]) / mix[3]));
            // green
            mix[1] = Math.round((added[1] * added[3] / mix[3]) + (base[1] * base[3] * (1 - added[3]) / mix[3]));
            // blue
            mix[2] = Math.round((added[2] * added[3] / mix[3]) + (base[2] * base[3] * (1 - added[3]) / mix[3]));

        } else if (added) {
            mix = added;
        } else {
            mix = base;
        }
        base = mix;
    }

    return mix;
}

// outputs [85, 170, 0, 0.75]
console.log(blendColors(
    [0, 0, 0, 0],
    [255, 255, 255, 0],
    [255, 0, 0, .5],
    [0, 255, 0, .5]
));