Even on the issue of accelerating and decelerating to a system you fly through or near, in the case of a colony ship you would need to carry that fuel and pay t

Even on the issue of accelerating and decelerating to a system you fly through or near, in the case of a colony ship you would need to carry that fuel and pay the energy costs for housing it, accelerating it and decelerating it. With a rogue planet you should be able to refuel from that rogue body so your fuel payload required is really only to get to the rogue planet.