Why does my NXT colour sensor not recognise colour on my iPhone?
The robot in the video has a custom light sensor, a more advanced HiTechnic color sensor V2 which can detect an extended range of 15 colours. The stand colour sensor can detect six (white, black, red, blue, green and yellow).




  The sensors are based on different principles. LEGO sensor uses a RGB
  LED, and successively shines red, green and blue light on the object.
  Reflected light is collected by a light sensor sensitive to all
  wavelength. HiTechnic one has a white LED, and a specialized color
  sensitive chip. This chip has three sensitive areas covered by red,
  green and blue filters. An added advantage of HiTechnic method is that
  it can detect the color of light sent on it (in passive mode, with the
  white LED shut off).


[Source]

A direct LED light against an iPhone's screen will reflect the light back into the sensor, making it difficult for the sensor to detect colour.