A bidding system is a collection of conventional agreements to be used. A system should cover all frequent bidding sequences, including all opening bids, responses to opening bids and all overcalls.
- A natural is green
- A system using 1♣ or 1♦ as artificial strong opening (usually showing 15/16/17 or more HCP) is blue
- A system that contains a convention that falls into the highly unusual method category is yellow(HUM)
- A system that does not fall into the above 3 categories is red