Dynamically created SVG image. Inspired by https://twitter.com/newsycombinator/status/967806990092242944 and http://blog.excites.co.uk/post/161030046411/rainbow.

spiral.svg

<svg onload="init()" xmlns="http://www.w3.org/2000/svg"
aria-label="Calendar" role="img"
viewBox="0 0 512 512">
<script type="text/ecmascript"><![CDATA[
//
// https://stackoverflow.com/questions/5736398/how-to-calculate-the-svg-path-for-an-arc-of-a-circle
//
function polarToCartesian(centerX, centerY, radius, angleInDegrees) {
  var angleInRadians = (angleInDegrees-90) * Math.PI / 180.0;

  return {
    x: centerX + (radius * Math.cos(angleInRadians)),
    y: centerY + (radius * Math.sin(angleInRadians))
  };
}

function describeArc(x, y, radius, startAngle, endAngle){

    var start = polarToCartesian(x, y, radius, endAngle);
    var end = polarToCartesian(x, y, radius, startAngle);

    var largeArcFlag = endAngle - startAngle <= 180 ? "0" : "1";

    var d = [
        "M", start.x, start.y,
        "A", radius, radius, 0, largeArcFlag, 0, end.x, end.y
    ].join(" ");

    return d;
}

function drawArc(x, y, radius, start, end, color) {
  var arc = document.createElementNS("http://www.w3.org/2000/svg", "path");
  arc.setAttribute("fill", "none");
  arc.setAttribute("stroke-width", "15");
  arc.setAttribute("stroke", color);
  arc.setAttribute("stroke-linecap", "round");
  arc.setAttribute("d", describeArc(x, y, radius, start, end));
  document.getElementsByTagName("svg")[0].append(arc);
}

function drawSpiral(startAngle, x) {
  var color;
  var startHue = Math.round(Math.random() * 256) % 255;
  for(var r = 20; r < 254; r += 20) {
    color = "hsl(" + Math.round((startHue + r) % 200) + "," + x + "%, 50%)";
    startAngle += 20;
    drawArc(250,250,r,startAngle % 360,(startAngle + 90) % 360,color);
  }

}

function init() {
  // remove warning since JS works
  var warnings = document.getElementsByTagName("text");
  for(var i = 0; i < warnings.length; i++) {
    warnings[i].setAttribute("font-size", "0");
  }
  // Do the magic
  var initialAngle = Math.round(Math.random() * 360);
  drawSpiral(initialAngle, 50);
  drawSpiral(initialAngle + 90, 70);
  drawSpiral(initialAngle + 180, 90);
  drawSpiral(initialAngle + 270, 100);
}
]]></script>
  <text x="40" y="30" font-family="Verdana" font-size="20">Need JavaScript support for this to display.</text>
  <text x="50" y="50" font-family="Verdana" font-size="20">Open file in Chrome / Firefox / Safari.</text>
</svg>

To see the image you need to download the SVG file and open it on a browser (Chrome, Firefox or Safari will do). Every time you open it the colors will vary.