Overview of Colorado 2 Club opening
Opening 2♣ shows a balanced hand with 18-19 HCP.
Why do we adopt this bid?
We have a couple of reasons
1) Did you every open this strong of a hand with 1 of a minor, intended to jump rebid at 2NT, and your partner passed you? Annoying right? The most common continuation is that your RHO will balance in the auction and you can now bid 1NT and hopefully still play the contract. However, you have allow your opponents to communicate and they might decide on a better choice, for them, with a 2-level sacrifice (or maybe even a makeable contract)
2) We notice that most 18-19 HCP hands, even when the opposite hand has 2-4 HCP (passing if opening 1 of a minor) still have solid chances at taking 8 tricks in No-Trump.
3) Balanced, 18-19 HCP hands could be consider 1 ½ NT. Too strong to open 1NT and too weak to open 2NT. However, wouldn't you still like to play your favorite NT responsive conventions (Transfers, Stayman). Colorado 2♣ allows us to do that.
Responding to the opening 2♣
Since the bid can be considered akin to opening 1 ½ NT, we use transfer responses.
2♦ requests the opener to bid hearts. It promises 5 hearts
2♥ requests the opener to bid spades. It promises 5 spades
2♠ requests the open to bid 2NT. This bid ensures that the opener will declare if the ultimate contract is No-Trump. This allows the responder to bid 3♣ over the 2NT relay as Puppet Stayman (game values, asking about the opener's major suit shape)
Very rare occurance: the responder can pass the 2♣ bid when they have <6 HCP and 5+ card club suit. In their opionion 2♣ will be safer than playing 2NT. Quite often the opponent's are at a lost after the 2♣ is passed and allow it to play, missing out on finding a major suit fit at the 2-level.
Responding 3♣ to the 2♣ opening is a game force bid with both majors. Responding 3♦ shows a very weak hand, but with at least 9 cards in the major suits. Opener should pick which major they like and play the partscore.
Another advantage is that Texas Transfers are also available. Bidding 4♦s and 4♥s are transfers and the responder is now in a position to cue-bid when interested in a slam (or bid 4NT Aces Asking).
Do we give up anything with this convention?
We can no longer use the 2♣ opening for all strong hands (bigger than 2NT opening or 9 sure tricks in a self-supporting trump suit). Not a problem, we'll use the 2♦ opening bid to show these hands. The weak 2♦ opening is not very useful as a preemptive tool, often gets overbid by opponents with majors. This is presented in the fact that many expert pairs replace the opening 2♦ with other conventions like Flannery, Multi, Mini-Roman, etc.
See the overview_milehigh2D for more information on the MileHigh 2D strong bid.