Cavgunner, my colony defense philosophy is a hierarchy:
1) Scout first. All systems have some kind of sensors on every jump point, either a picket ship, sensor platform, or sensor buoys.
2) All inhabited colonies have a small PDC carrier to base scouts on. These can probe contacts, shadow an enemy, and even if the colony is destroyed, hopefully they can shadow the invaders back to where they came from to localize their jump entry.
3) Any colony with a significant population and industry should at least have some protection from a random missile attack. This can be from planetary AMM PDCs, which can be quite efficient as you can get 5 second reloads easily, and you can use colony missile stocks, so you don't need to spend on magazines. Another option is railgun orbitals. Basically you stick a whole bunch of base tech railguns in orbit, they cost 1 BP each, and use your race's base tracking. You can even under power them for extra cost savings. Because all their systems are so cheap, most of the time, a maintenance failure is going to be cheap to fix, so you can have a very long endurance on them for little cost. They provide enormous PPV for their cost, can deal with box launcher volleys quite well, and soak an enormous amount of damage for their cost.
So any colony worth protecting can have a bunch of 10,000 ton orbitals that cost about 300 BP each. It is all about cheaply mitigating the damage an attack can do. You won't be able to protect the shipping, especially the dumbass civilians, no matter what you do. But early detection allows for a quicker response. And missile defenses means that the colony itself will generally not take much damage because the enemy is likely to shoot all its missiles on approach.