Adding on that, in general and in game design (if you read some of the websites talking of that, like Gamasutra.com) it is generally a good idea to provide a tool for the player (even if a difficult to use tool) for most if not all of the problems he encounters.
So what I'm saying is if a player want to help a colony, then adding the possibility to strengthen it directly would be nice, gameplay wise.
Now, Colony Defense can get huge as we know, given that the coefficients multiply between themselves. So what about something that is really significant for small to medium colonies, where defense is between 50 and 1000? Adding to this defense, as a flat value, outside of any multiplier the number of Marines in the 'Open Market' ONLY would be a nice little boost.
So for example I see a weak colony of my friend from the Hegemony is at 150 and will be raided. I add 200 Marines to the open market, and now it is at 350. And as that's a flat value, I can't get to stupid values. Remember that a Military World can go to 10.0000, so we should be safe here, while providing a nice little tool to the player.