title: In show touches of Android, What does the color lines mean
body: <p>When I turned on the debug mode.Set "USB debugging",and turn on "Show touches".It should show lines when I touch in the screen.
  <img src="https://i.stack.imgur.com/mhDnT.png" alt="enter image description here"></p>
  
  <p>Could you tell me what is the meaning of the lines' color.
  The blue line is ..?
  the red line is ..?
  and the purple is ...?</p>