Just looked up the leislation in relation to this and it says
(Regulations 18 and 20)
SCHEDULE 8
requirements relating to obligatory and optional hazard warning signal devices
Every hazard warning signal device shall–
(a) be operated by one switch;
(b) cause all the direction indicators with which a vehicle or a combination of vehicles is equipped to flash in phase;
(c) be provided with a circuit-closed tell-tale in the form of a flashing light which may operate in conjunction with any direction indicator tell-tale; and
(d) be able to function even if the device which controls the starting and stopping of the engine is in a position which makes it impossible to start the engine.
Don't know if the MOT regs are different but it seems we have the thumbs up from Her Madge it appears! Don't mention anything about the colour of it...