Okay, got some more info for you. Everybody paying attention?
Looking into it, it would seem that the multi-coloured flag with the crescents actually
pre-dates the plain red one. It is the flag of the Omani Ya'rubi dynasty.

This older version had a triangular cut-out. The Ya'rubi were replaced by the Al-Busaid (Al-Bahaid?) dynasty in the 18th Century and these guys got rid of it and replaced it with their own red flag.
The old 17th-18th Century Ya'rubi flag was reintroduced (without the cut-out) at some point in the second part of the 19th Century once Zanzibar was independent, but the red flag continued in use too.
The British protectorate established in 1890 briefly used this flag:

But then for some reason soon abandoned it and went back to the plain red one:

Seeing as the period of existence of the Zanzibari regulars spans the use of all three flags, in the absence of more info to the contrary it would appear that gamers could justify using any of them with their figures, according to taste and time-frame.