How do I pick one specific object on a collision

I am testing if two objects of the same container collide, but I wish to pick one of them specifically. For instance:

Block: overlaps Block : offset (16, 0)

In this event, I wish to set a private variable of Block in terms of the ID of Block.

Any suggestions?

put them in a family

let's say green

block overlaps green

---block - do this

---green - do that

thanks a lot.