Best when black has played e6 blocking in the bishop. If black's bishop is on f5 or g4 then a colle is not so good.
Also a major point of the colle is the pawn break on e4... so it's not so good against setups where black hasn't played d5. So a KID type setup for example (d6 and g6) will make a colle look silly (you should play c4 and/or e4).
Just a random question I decided to throw out there: what do you think of the colle system?