I don't remember exactly off the top of my head how many M16A3 contracts FNMI had, and when - but you might be able to look at the 5.56MM Timeline and get a range of dates of contracts.
FNMI's M16 production has had a couple of different anodizing variations, ranging from a "Sporter Gray" that had a bluish cast to it, to black.
FWIW, again - off top of my head, without looking at it again, only FNMI and Sabre (albeit briefly) have manufactured production M16A3s, though Colt manufactured and delivered the initial M16A2E3s (which were so marked), I don't think they ever got an "A3" contract - by then I think they had largely shifted their focus to the M4 family anyways.
Actually, though I suppose you may have remembered it more distinctly if it were the case - but there's an extremely high likelihood of purple in the mix here, as it matches the general time-period of the bulk, IIRC, of M16A3 production.
As a "rule of thumb" you're looking at going from lighter to darker in terms of anodization, though that's not an ironclad rule.
~Augee