Been a while since I looked up the site...can help you out on MARTEL, did a Q on it back in the day when I worked on Buccs.
The fairing at the back of the missile is a part of the missile structure and the 5 'holes' are where the rocket motor venturies protruded from. The missile had two motors, a 'booster' and a 'sustainer', the large hole in the centre was for the larger sustainer motor venturi, the booster motor was a tubular shape with the vent to the sustainer passing through the centre, and had four smaller venturies, hence the four smaller holes. As far as I can remember they were all just inside the skin of the fairing.
As for colour a lot of the training rounds fitted were the same off white colour depicted in the second picture (the pink band denoting a training round), though there may have been the odd green one kicking about. The only training rounds were of the anti-radar version, as the TV missiles were simulated by fitting a TVAT (TV Airbourne Trainer) pod. The missile at Cosford is probably one of the many drill rounds we used to use on the old generation exercises, as it would have been a bit silly loading live stuff and exposing it to the wind/snow/rain etc. hence the lack of any venturies. If memory serves me right the ends of all the venturies were protected from the elements by a thin plastic membrane that was off white in colour, which fitted perfectly into the holes in the rear fairing, but I could be wrong on that one.
In my day all live rounds were the same glossy green as seen in the photo (deep bronze green), should think if they were off white they didnt stay like that for long. The AR missile had one yellow band on the warhead...similar location to the pink band seen on the training round, and a brown band where you see the blue band on the top photo, and perhaps a second brown band a bit further up by the wings. The TV missile had similar banding, and a black band just behind the yellow one (TV warhead was different).
The launcher differs as well, the front of the AR launcher seen in the picture is removed and a larger bit went in its place, this housed a bottle of inert gas which flowed into the TV head via a 'frangible pillar' (yellow), basically a bit of pipe that detached when the missile fired, if the MARTEL in the museum is a TV one you'll see the position the pillar was fitted to quite easily.
Been a while since I played with them, and I'm not sure if I have any pictures (doubtfull), but if anthing else crops up in my dusty old brain or if I find anything out I'll let you know.