Return a list of colors([r, g, b]) that transit from/to all colors(hex) in the list.

demo.js

spectrum = getSpectrum(20, ['#2E7D32', '#ffee58', '#B71C1C']);
console.log(spectrum);
// [[46,125,50],[67,136,54],[88,148,58],[109,159,61],[130,170,65],[151,182,69],[171,193,73],[192,204,77],[213,215,80],[234,227,84],[255,238,88],[248,217,82],[241,196,76],[233,175,70],[226,154,64],[219,133,58],[212,112,52],[205,91,46],[197,70,40],[190,49,34]]

/* we can apply the color to css */
$('div').css('background-color', 'rgb(' + spectrum[3] + ')');

getSpectrum.js

function getSpectrum(n, colors){
  function hexToRGB(h){
    var hexToDec = function(h){
      return parseInt(h, 16);
    };
    h = (h.charAt(0) == "#") ? h.substring(1, 7): h;
    rgbHex = [h.substring(0, 2), h.substring(2, 4), h.substring(4, 6)];
    return rgbHex.map(hexToDec); 
  }
  
  function _getSpectrum(n, from, to){
    // calculate the step for r/g/b.
    var step = [];
    var step_acc = [];
    for(var i = 0; i <= 2; i++){
      step[i] = (to[i]-from[i])/n;
    }
    
    // generate n colors, i.e. a spectrum
    var spectrum = [];

    for(var i = 0; i < n; i++){
      var color = [];
      for(var j = 0; j <= 2; j++){
        color[j] = Math.round(from[j] + step[j] * i);
      }
      spectrum.push(color);
    }
    
    return spectrum;
  }
  colors = colors.map(hexToRGB);
  var spectrum = [];
  var m = Math.floor(n / (colors.length - 1));
  var acc = n;
  for(var i = 0; i < colors.length - 1; i++){
    acc -= m;
    if(acc > 0 && acc < m){ m += acc; }
    spectrum = spectrum.concat(_getSpectrum(m, colors[i], colors[i+1]));
  }
  return spectrum;
}

You can see the live example at here.