if I remember right, if you order enough there is no charge for whatever colour you want within reasonDid a Pyro job once in a grey cable in a church ,that was specially ordered like , not sure about FP cable , I suppose you can get it in different colours because thats what you would use today in locations like this , but specially ordered of course and cost a bit too..
Is this a listed building?.... you may have to confirm your wiring methods and from experience if its exposed stone work and a cable has to run along it like in a chapel or church then bare MICC is usually the only acceptable method... shame they don't teach it anymore .... if listed don't choose the easy option without confirming you can or you may end up removing it all at your own cost!