interpolate.js

var hsv2rgb = function(h, s, v) {
  // adapted from http://schinckel.net/2012/01/10/hsv-to-rgb-in-javascript/
  var rgb, i, data = [];
  if (s === 0) {
    rgb = [v,v,v];
  } else {
    h = h / 60;
    i = Math.floor(h);
    data = [v*(1-s), v*(1-s*(h-i)), v*(1-s*(1-(h-i)))];
    switch(i) {
    case 0:
      rgb = [v, data[2], data[0]];
      break;
    case 1:
      rgb = [data[1], v, data[0]];
      break;
    case 2:
      rgb = [data[0], v, data[2]];
      break;
    case 3:
      rgb = [data[0], data[1], v];
      break;
    case 4:
      rgb = [data[2], data[0], v];
      break;
    default:
      rgb = [v, data[0], data[1]];
      break;
    }
  }
  return '#' + rgb.map(function(x){
    return ("0" + Math.round(x*255).toString(16)).slice(-2);
  }).join('');
};

/**
 * Return hexadecimal code between green and red
 */
module.exports = function(val) {
  if (val > 100) {
    val = 100;
  }
  else if (val < 0) {
    val = 0;
  }
  var h = Math.floor((100 - val) * 120 / 100);
  var s = Math.abs(val - 50)/50;
  var v = 1;

  return hsv2rgb(h, s, 1);
};