Thank you Saad Swad for the help, I succeeded what I wanted to do thanks to you.
I did as you asked and add some stuff for fun :
Event : when character on collision with purple sprite =>Action : set opacity of purple sprite to 50, set "Collision" (instance variable of purple sprite) to 1.
Event : Every 1.0 seconds,"Collision"(instance variable of purple sprite)=1, Pick a random purple sprite instance => Action : Apply Physics Force (0,500) at image point 0.
Is there a way to join the project capx or images in the Forum, then this could help more other people ?