How do I apply a force toward a different instance of the same object?

I'm trying to make a simulation with red and yellow particles that follow a few rules, one of those being that red particles are attracted to each other. I tried to do this using physics force, but the force is applied to itself instead of another instance. This results in all red particles moving to the right.

Is there a better way to do this?

Try "for each".

Also generally if you want two instances of the same object to affect each other, put the object in a family and use the object and family in the condition.