In my old article "Hard Candy" which was published here exactly one year ago (http://www.chess.com/article/view/to-be-named) I wrote that "some openings are very venomous despite their innocent appearance." The Colle System is definitely one of them. This variation was championed by Belgian master Edgard Colle. Due to his health problems and untimely death at the early age of 34 he couldn't demonstrate his full potential. That's why today most chess-players remember him only for his opening even though he was a strong player and had many outstanding results.
These days the Colle System is very popular in scholastic tournaments. It is understandable that it is so easy for most coaches to show their young students the opening moves (1. d4, 2.Nf3, 3. e3, 4. Bd3) which you can play practically against any Black set up. The problem is that many chess-players don't really understand the key ideas of the opening. So instead of trying to seize the initiative and start an attack at the first possible moment, they just enjoy their solid position and play natural moves. It is probably the main reason why the majority of chess-players consider it a safe but somewhat passive opening. But if it is such a passive opening, how could Edgard Colle beat the best Grandmasters of his time using this system? This is the list of outstanding players who were beaten by Edgard Colle in the Colle system: World Champion Max Euwe,World Title contender Efim Bogoljubov, the famous theoreticians and authors of their own openings Ernst Gruenfeld ( The Grunfeld Defense) and Vasja Pirc (The Pirc Defense), as well as famous GMs Akiba Rubinstein and Rudolf Spielmann. What was Colle's secret for turning a passive looking opening into a very dangerous weapon? I noticed that in most of his games he was not afraid to take risks and start his attack whenever he had a chance. One of his favorite moves was Ng5 which he played whenever possible. It is a well known saying about Tal's games: "If Tal has an open file, there will be a checkmate!" I think we can say the same thing about Edgard Colle and the Ng5 move :)
When a chess-player wants to learn a new opening he usually buys opening monographs, Encyclopedias or chess videos about the subject of his interest. Let me suggest a different way. If you want to learn the Colle System, just get the games played by the Master himself. You'll understand the spirit of the opening and learn the main ideas much better. Besides it is much more interesting than just read those opening books. Here I want to give you some tactical puzzles taken from the games played with the Colle System by Edgard Colle!
If you never played the Colle System before, do yourself a favor and try it at least once just out of respect for a great master of attack, Edgard Colle. I am telling you, you may love it!