Signage is used to provide information and advice to people relating to all aspects of health and safety and in all environments. The most commonly seen signs will be blue "Fire Door - Keep Closed" and green "Fire Exit" signs - these are present in all work places. THere are also a large number of more specific signs which will only be seen in certain environments.
Blue signs show actions that "MUST" be carried out - they are mandatory
Green signs show a "SAFE CONDITION" - such as an escape route or first aid box
Yellow signs are "HAZARD WARNING" signs - they provide information about specific hazards present in the locale
Red signs are "PROHIBITION" signs - they show actions that must NOT be carried out (e.g. "no entry")