Netlogo: How can I ask the specific patch changes the color (black) only for a specific ticks with the timing of Poisson distribution?

How can I ask the specific patch changes the color (black) only for a specific ticks with the timing of Poisson distribution? I am a beginner of Netlogo. The following is a sample program. But in this program the patch has been white colored all the time. Thank you.

ask patch max-pxcor 0 [
set poisson poisson - 1
if poisson < 0 [ ;I have no idea of the good condition setting.
  set poisson random-poisson (stop-ticks)
  set pcolor black 
]
  set pcolor white
]