code: function(r, g, b, a) {
      var rgb = [r, g, b].map(function (c) { return number(c); });
      a = number(a);
      if (rgb.some(isNaN) || isNaN(a)) return null;
      rgb.push(a);
      return rgb;
    }
docs: Given an rgba color as number-like objects, return that array
  with numbers if possible, and null otherwise
  
  @param {number} r red
  @param {number} g green
  @param {number} b blue
  @param {number} a alpha
  @returns {Array} rgba array