Mandatory signs are required to be blue. A mandatory sign shall indicate what action is required in a particular situation, place or circumstance. Mandatory signs usually display a white safety symbol on a blue background.
Mandatory safety signs are not, as is commonly perceived, always a requirement by law'. There are in fact 5 different types of safety sign - some are prescribed by law, others by simple common sense and good practice. It It largely depends on the situation and category of risk being referenced by their use.