contrast.js

var lum = function(hex) {
  var bits = hex.match(/([0-9a-f]{2})/ig).map(function(i){ 
    var it = parseInt(i, 16) / 255; 
    if(it < 0.03928)
      return it / 12.92;
    else
      return Math.pow((it + 0.055) / 1.055, 2.4);
  });
  return 0.2126 * bits[0] + 0.7152 * bits[1] + 0.0722 * bits[2];
};

var ratioTest = function(bg, fg) {
  return (lum(bg) + 0.05) / (lum(fg) + 0.05) > 4.5; 
};