code: function blend(top, bottom){
    // TODO: different blend modes like overlay etc.
    utils.assertColor(top);
    top = top.rgba;
    bottom = bottom || new nodes.RGBA(255, 255, 255, 1);
    utils.assertColor(bottom);
    bottom = bottom.rgba;
  
    return new nodes.RGBA(
      top.r * top.a + bottom.r * (1 - top.a),
      top.g * top.a + bottom.g * (1 - top.a),
      top.b * top.a + bottom.b * (1 - top.a),
      top.a + bottom.a - top.a * bottom.a);
  }
docs: Blend the `top` color over the `bottom`
  
  Examples:
  
  blend(rgba(#FFF, 0.5), #000)
  // => #808080
  
  blend(rgba(#FFDE00,.42), #19C261)
  // => #7ace38
  
  blend(rgba(lime, 0.5), rgba(red, 0.25))
  // => rgba(128,128,0,0.625)
  
  @param {RGBA|HSLA} top
  @param {RGBA|HSLA} [bottom=#fff]
  @return {RGBA}
  @api public