They don't need to. They just need rockets, lots and lots of them. All colonists need is to deny the incoming party the ability to make a successful landing or orbit.. Unless you want to nuke or slam an asteroid into Mars, conventional warfare simply do not work well on the timescales of space travel. Another alternative for the red team is stealth, but our current understanding of metamaterials is nowhere advanced enough for an invisibility coating in the optical spectrum that can hide a spacecraft capable of interplanetary travel. Perhaps they are sufficient for a missile that only needs to make a minimal adjustments to direction.