RAINBOWlog.js

(function(){
var log = console.log;
var css = 'background: linear-gradient(to right, red, yellow, lime, aqua, blue, fuchsia, red); color: white; font-weight: bold; text-shadow: -1px 0 black, 0 1px black, 1px 0 black, 0 -1px black;';

function gen_css(){
  return css + "font-size: " + (9 + ((Math.random() * 20) << 0))  + "px;"
}

console.log = function(str) {
  var args = Array.prototype.slice.call(arguments);
  var letters = args[0].split("");
  var css = letters.map(gen_css);
  args[0] = "%c" + letters.join("%c");
  args.splice.apply(args,[1,0].concat(css));
  return log.apply(console, args);
}
})();

added random height letters