A "mandatory sign" means a sign prescribing behaviour. These signs are required to be blue. A mandatory sign shall show only what action is required. Mandatory signs generally use a white safety symbol on a blue background. Mandatory safety signs are not, as some people believe, 'those which are required by law'. There are 5 different types of safety signs - which type is 'required' by law or by common sense - depends on the situation and type of risk being addressed by their use