Safety signs are regulated by shape and colour so that meaning survives language, distance and smoke. For fire purposes there are four categories in general use, and the reason a sign looks the way it does is that the shape carries the category before the pictogram carries the detail.
The four categories
Prohibition signs are round, red bordered with a diagonal bar, and forbid something: no smoking, do not use lift in event of fire. Warning signs are triangular and yellow, and identify a hazard: flammable material, explosive atmosphere. Mandatory signs are round and blue, and require an action: fire door keep shut. Safe condition signs are rectangular and green, and identify escape routes, exits and first aid. Fire equipment signs are rectangular and red, and identify where equipment is.
Why the shape matters more than the words
Shape and colour are readable at a distance and through smoke long after text has become unreadable, and they work for people who do not read English. Regulations require signs to comply with the specified shapes and colours precisely so that this holds, and a well meant custom sign in the wrong colour breaks it.
The two that get confused
Green is safe condition and red is fire equipment, and both are rectangular. Green tells you where to go. Red tells you where a thing is. A green sign over an extinguisher and a red one over an exit would be a serious error, and it happens in buildings where somebody has bought signs by appearance.
Supplementary text and arrows
Text below a pictogram is supplementary and is there to make the pictogram unambiguous rather than to replace it. Directional arrows on green signs must be consistent along a route, because a single arrow pointing the wrong way at a junction undoes the whole run.
Questions people ask about which of the following are categories of fire signage
Is fire equipment signage green or red?
Red, rectangular, with a white pictogram. Green rectangles are safe condition signs for escape routes and exits.
Are photoluminescent signs required?
They are required where the sign must remain visible if the lighting fails and emergency lighting does not cover it. Many buildings use them alongside emergency lighting rather than instead of it.
Can a building use its own sign designs?
Not for the regulated categories. The shapes and colours are specified, and a bespoke design in the wrong shape does not meet the requirement however clear it looks in daylight.