foo

foo.js

var nodes = document.querySelectorAll('.day');
var items = ['#d6e685', '#8cc665', '#44a340', '#1e6823']

Array.prototype.randomElement = function () {
    return this[Math.floor(Math.random() * this.length)]
}

for (var i = 0; i < nodes.length; i++) {
	if (nodes[i].getAttribute('fill') == '#eeeeee') {
  	nodes[i].setAttribute('fill', items.randomElement());
  }
}