I think the colour coding just give a general description of the system as mentionned in the classification of system and the adding of a special convention doesn't mean that a system can pass from, for example, green to blue : Classification of Systems In order to facilitate recognition and handling, systems material will be identified by one or more of the following: a WBF coloured sticker; the appropriate name (hand printed or typed) colour; a check mark on a convention card next to the appropriate colour - in keeping with the following descriptions: Green Natural Blue Strong Club/Strong Diamond, where one club/one diamond is always strong Red Artificial: this category includes all artificial systems that do not fall under the definition of Highly Unusual Methods (HUM) systems [see definition below], other than Strong Club/Strong Diamond systems (see 'Blue'). Examples would be a system where one club shows one of three types - a natural club suit, a balanced hand of a specific range, or a Strong Club opener; or a system in which the basic methods (other than the no trump range) vary according to position, vulnerability and the like; or a system that uses conventional 'weak' or 'multi-meaning' bids (with or without some weak option) in potentially contestable auctions, other than those described in the main part of the WBF Convention Booklet Yellow Highly Unusual Methods ('HUM') as defined above.