As you may know, the British Beer and Pub Association (BBPA) & Society of Independent Brewers (SIBA) maintain an informal container colour band registration scheme on behalf of UK brewers.
http://www.beerandpub.com/cask
If you are a new brewery yet to order containers, please go to the database and search for available colours. If they don't come up with a brewery, then they have not been registered. This does not guarantee that no one is using them, but we can only go by those registered colours.
Please contact containers@siba.co.uk to register your container colours, and notify us of previous owners if applicable