gistfile1.js

var highest = 16777215;
var steps = 1000;
var step = highest / steps;
var emptyColor = "000000";

for (var i=0; i < steps; i++) {
  var color = parseInt(step * i, 10).toString(16);
  color = emptyColor.slice(0, -color.length) + color;
  var span = document.createElement("span");
  span.innerHTML = "a"
  span.style.color = "#" + color;
  document.body.appendChild(span);
}