The Department of Transport states that the UK preference of colours is yellow to the side and red to the rear.
There are two options to choose from, a complete contour line marking or a partial contour marking. Partial contour markings are the minimum requirement. Contour markings should cover at least 80% of the total length and width of the vehicle/trailer and should be as close as practicable to the edges. A combination of marked and unmarked vehicles/trailers is discouraged.