Javascript - "One shade darker"

Is it possible to use javascript to determine what color is one shade darker than the current background? Maybe some hexadecimal addition/subtraction?

I have a menu that can be any color and if it wasn't too d开发者_StackOverflowifficult it would be great if the submenu could be one shade darker. Does anyone know how to achieve this effect?

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Something like this:

function shadeColor(color, shade) {
    var colorInt = parseInt(color.substring(1),16);

    var R = (colorInt & 0xFF0000) >> 16;
    var G = (colorInt & 0x00FF00) >> 8;
    var B = (colorInt & 0x0000FF) >> 0;

    R = R + Math.floor((shade/255)*R);
    G = G + Math.floor((shade/255)*G);
    B = B + Math.floor((shade/255)*B);

    var newColorInt = (R<<16) + (G<<8) + (B);
    var newColorStr = "#"+newColorInt.toString(16);

    return newColorStr;
}

Usage:

var newColor = shadeColor("#AA2222", -10);
alert(newColor); //Results in #a32020

Here is an example code to test it: http://pastebin.com/g6phySEv

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as AB comments, 'shade' isn't very well defined. nonetheless, it might be easier to think of this in some other colour representation, such as 'V' in hsv.

you could either convert, decrease v and convert back, or figure out what decreasing v maps to in rgb hex