It does not have to be intumescent paint but it should not affect the performance of the door. Avoid using oil based paints and varnishes where possible. Flammable materials should be avoided. The issue may not be the flammability of the substance used but rather the bubbling when heat applied which may affect a persons escape. Water based gloss or eggshell.

The internal linings should be class O. Paint is not an internal lining and provided that the fire door and walls etc are Class 0 then the integrity of the internal linings are complete and in compliance with the building regs. tech guidance doc. B.
Therefore as i had said if you stick to waterbased paints, the likelihood of them affecting the internal lining of a protected escape route are pretty much non existent. You're pretty much back to the kids handing drawings on school corridor scenario.

Agree with Kev's advice. Intumescent paint has quite specific and unusual applications. We used it for example on old listed doors with cracks in them in residential premises. When painting fire doors please be careful on smoke seals and intumescent strips; one of my team did some research on this and he believes you can have up to 5 coats of paint on an intumescent strip, but I guess no coats would be best,

John

BS 8214:

'Unglazed areas of any fire door leaves are generally not required to provide a specific surface spread of flame requirement and may therefore be decorated as desired.'

The BS also goes on to state that Intumescent seals can be painted up to 5 coats (regular oil-based paint) and some paint is probably advantageous to control moisture absorption.

(jwk - your Team could have saved a lot of time by simply reading the BS!).

There was a thread not too long ago about wallpapering fire doors. That was just as much fun as this one.

Please don't start this again!