Is their a way I can make the circles change color when they are getting close to each other to a certain distance?

Hi! I'm really new to grasshopper, I'm trying to do a particle simulation of pedestrian flow by kangaroo. Is there a way I can manage the above task of the title? Really appreciate if you could help me.

I'm trying to do what Saied Zarrinmehr had done in his work of 

Simulating Dynamical Features of Escape Using Grasshopper Physics E...   

The above is a link to his work and video.

This is what I have got right now, if anybody could help me to improve it, I will be very appreciate for it, too.