Connecting objects on collision

Hi, I'm quite new to Unity and I'm struggling with connecting objects when they collide. I have boxes falling on each other of different colours, and I want the ones of the same colour to attach to each other.

I've tried using a fixed joint and setting two public game objects. However because it always spawns a clone of the prefab I think it's connecting to itself?

Not sure where to go with this and any help would be greatly appreciated! Thanks