From the MCA Website, as the yellow codes says, its for small commerical vessels, you must decide if you are operating commerically or not. Worth buying the Yellow book if you are thinking of going down this route.
Pete
Code Vessel Safety
Small Vessels operating commercially under the British Flag or in British waters must comply with the Merchant Shipping Regulations or an MCA Code of Practice. There are currently four codes for Small (up to 24m) Vessels.
YELLOW
The Code of Practice for the Safety of Small Commercial Motor Vessels
BLUE
The Code of Practice for the Safety of Small Commercial Sailing Vessels
BROWN
The Code of Practice for the Safety of Small Workboats and Pilot Boats
RED
The Code of Practice for the Safety of Small Vessels in Commercial Use for Sport of Pleasure operating from a Nominated Departure Point (NDP)
There is also a Code of Practice for commercially operated yachts 24m and over