There were at least two OEM suppliers of exhaust systems to Triumph: Harmo and Burgess are the two that I know of. I have seen original and replacement exhaust bits from both suppliers in (pretty much gloss) black and (somewhat flat) silver. It's certainly easy enough to find high-temp black engine paint (probably silver, too?)
Point of reference: I used to think that all Harmo systems were black...until I got two NOS early Herald rear mufflers a month or so back. Harmo brand, painted silver. For ease and convenience, as well as protection, one of those will get cleaned up and sprayed black before it goes on my Herald.