gistfile1.js

// get the average color of two hex colors.
function avgcolor(color1,color2){
    var avg  = function(a,b){ return (a+b)/2; },
        t16  = function(c){ return parseInt((''+c).replace('#',''),16) },
        hex  = function(c){ return (c>>0).toString(16) },
        hex1 = t16(color1),
        hex2 = t16(color2),
        r    = function(hex){ return hex >> 16 & 0xFF},
        g    = function(hex){ return hex >> 8 & 0xFF},
        b    = function(hex){ return hex & 0xFF},
        res  = '#' + hex(avg(r(hex1),r(hex2))) 
                   + hex(avg(g(hex1),g(hex2))) 
                   + hex(avg(b(hex1),b(hex2)));
    return res;
}

// e.g.
avgcolor('#ffffff','#000000'); // "#7f7f7f"