/**
 * Determine the mean value of two CSS color, where mean is by channel
 * @param {string} color1   string of the first color
 * @param {string} color2   string of the second color
 * @requires {string}       string of the mean color
 */
var average_color = function(color1, color2) {
    // Assumption: "color1" and "color2" need not to be validated

    var r1 = parseInt(color1.substr(0, 2), 16),
        g1 = parseInt(color1.substr(2, 2), 16),
        b1 = parseInt(color1.substr(4, 2), 16);
    var r2 = parseInt(color2.substr(0, 2), 16),
        g2 = parseInt(color2.substr(2, 2), 16),
        b2 = parseInt(color2.substr(4, 2), 16);
    var r3 = Math.round(0.5 * (r1 + r2)),
        g3 = Math.round(0.5 * (g1 + g2)),
        b3 = Math.round(0.5 * (b1 + b2));
    return (r3 * 65536 + g3 * 256 + b3).toString(16);
}; // var average_color = function(color1, color2) { ... };

console.log(average_color('135246', 'acfbe0'));