I have a hex color, e.g. #F4F8FB (or rgb(244, 248, 251)) that I want converted into an as-transparent-as-possible rgba color (when displayed over white). Make sense? I'm looking for an algorithm, or at least idea of an algorithm for how to do so.

For Example:

rgb( 128, 128, 255 ) --> rgba(   0,   0, 255,  .5 )

rgb( 152, 177, 202 ) --> rgba(  50, 100, 150,  .5 ) // can be better(lower alpha)

Ideas?

FYI solution based on Guffa's answer:

function RGBtoRGBA(r, g, b){

    if((g == null) && (typeof r === 'string')){

        var hex = r.replace(/^s*#|s*$/g, '');

        if(hex.length === 3){

            hex = hex.replace(/(.)/g, '$1$1');

        }

        r = parseInt(hex.substr(0, 2), 16);

        g = parseInt(hex.substr(2, 2), 16);

        b = parseInt(hex.substr(4, 2), 16);

    }



    var min, a = (255 - (min = Math.min(r, g, b))) / 255;



    return {

        r    : r = 0|(r - min) / a,

        g    : g = 0|(g - min) / a,

        b    : b = 0|(b - min) / a,

        a    : a = (0|1000*a)/1000,

        rgba : 'rgba(' + r + ', ' + g + ', ' + b + ', ' + a + ')'

    };

}



RGBtoRGBA(204, 153, 102) == RGBtoRGBA('#CC9966') == RGBtoRGBA('C96') == 

    {

       r    : 170,

       g    : 85 ,

       b    : 0  ,

       a    : 0.6,

       rgba : 'rgba(170, 85, 0, 0.6)' 

    }