Which plan for Black's c-pawn in Colle System?

I have run into this situation several times in recent months...

White plays the Colle System with his pawn at c3, and as Black I play ...c5. But what should I do with that pawn if White doesn't capture that pawn?
Should I then capture via ...cxd4, or should I advance it via ...c4, or maybe something else? Maybe I should play ...c3 instead in order to match his pawn position?

I ran into trouble in one game when I traded my c-pawn via ...cxd4 because my opponent got his bishop (at d3) and queen (at c2) aligned on the "Classic Diagonal" (b1-h7),
aiming at my h7 pawn while moving his KN in to join the attack. In the next game I had with him, I prevented him from moving his bishop to d3 by advancing
my c-pawn via ...c4, and I got a better game, but I don't know if that's the best move because White can rid himself of it via b3, which opens up the b-file for White to attack.

While I preach that reverse openings are not to White's advantage, a few, though not all, of the ideas remain the same.  In essence, the Colle Koltanowski (Colle with c3 rather than b3) is very similar to White playing a reversed Semi-Slav, and the Phoenix Attack, where White eventually takes on c5 and then plays b4, is in essence just that, a reversed semi-slav.

Black is behind a move compared to the Meran, so Black can't just take White's ideas in those lines and run with them.  That said, the solution for the c-pawn is still the same.  Usually Black wants to leave the tension on the board, especially early on.  White never wants to take on c5 before Black moves his Bishop (make Black waste a tempo).  Advancing to c4 is a grave mistake, and taking on d4 too early gives White the c3-square for his Knight, so if Black ever is going to take, which usually won't be until at least move 10 or so, he needs to wait until White has committed his Queenside Knight to d2.

So in essence, the answer is neither trade nor advance.  Keep the tension on the board until there is a specific reason to take, then it's more likely that taking will be more beneficial than advancing, though every scenario is different (i.e. If White puts his Bishop on d3 and Knight on b3, then surely I'll advance!)

Good answer, thanks ThrillerFan. I don't know the Semi-Slav but at least that gives me something specific to look at. A big part of the problem and the reason I couldn't just find my answer in an opening database as I usually do is that the Colle System is a *system*, not an *opening*, therefore the specific move order isn't very important, and the answer will be spread out across many database entries for different move orders.

A basic idea in chess is that you usually want more pawns in the center because pawns in the center give you more space. So a really crude  idea after 1.d4 d5 is that White would like to get rid of Black's d5 pawn and gain more space while Black would like to get rid of White's d4 pawn and do the same. After all it will be hard for Black to put his pawn on e5 if that d4 pawn is in the way. 2.c4 is a popular way of beginning this idea for White and then Black needs to find a plan of his own. The idea is similar after 1.e4 e5 but there moves like 2.f4 and 2.d4 have drawbacks and so White usually chooses 2.Nf3 to pressure Black's pawn and so on. Neither side usually wants to give up their center pawns without good reason (the pieces can create exceptions and even plans based on allowing such things to happen) and so both sides find other ways to pressure the center of the board. 

The Semi-Slav structure is solid but can be passive if that side does nothing because the opponent will be trying to use the strategy of "undermining pawns" to try and gain space. When White plays 1.d4 d5 2.c4 it usually isn't enough to make Black move the d-pawn and so White would like to play e4 to put even more pressure on that pawn. Tactics aside if this happens things can go well for White. Black in the Colle (reversed Semi-Slav) is usually on the reverse side of this situation but is going second and so has to be careful not to be too ambitious with this idea.

So now we know that Black is playing ...c5 to try and undermine the d4 pawn. We also know that White probably likes that pawn and won't move it (although he could). So Black needs to find other ways to bring pressure there. And in this Semi-Slav structure Black usually looks to play ...e5 as well as ...c5 to put even more pressure on d4. If this pawn leaves the board there will be a vacuum of sorts and play will depend on whose pieces can take advantage of the new space. But all that depends on tactics and calculation. In the mean time White will probably be trying to mate you on h7 and so things get a bit more complicated because both sides will be trying to get their plans to work.