Get black or white color depending on background color for best readability. Based on the YIQ color-space contrast ratio.

blackorwhite.js

function blackOrWhite(hexcolor) {
    var color = hexcolor.substring(1);
    hexcolor = color.length < 5 ? color + color : color;
    var r = parseInt(hexcolor.substr(0, 2), 16);
    var g = parseInt(hexcolor.substr(2, 2), 16);
    var b = parseInt(hexcolor.substr(4, 2), 16);
    var yiq = ((r * 299) + (g * 587) + (b * 114)) / 1000;
    return (yiq >= 125) ? '#333' : '#eee';
}