RAINBOWlog.js

(function(){
var log = console.log;

console.log = function(str) {
	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;';
	var args = Array.prototype.slice.call(arguments);
	args[0] = '%c' + args[0];
	args.splice(1,0,css);
	return log.apply(console, args);
}
})();

for firefox too, please :/