I must respectfully disagree with the very learned John the OFM. Provincial troops (not militia) were not bound by the British army regulations at all, and there is more speculation than known facts on Provincial flags.
Provincial units came and went during the time period of 1754 to 1760. For instance the original Virginia regiment had red coats and no facings (that is, if they even had a uniformed issued to them), but the later verision of this provincial unit had blue coats with red facings. There are indeed colours hanging in the Governor's Palace at Colonial Williamsburg that are supposed to represent th e later regiment (well, two actually), but who knows how they are documented, or for what time period they represent.
Many other colonies fielded provincial regiments as well, but again, they tended to come and go in establishment. The design of the colours would really be up to the Governor (as commander in chief of his colony), and or perhaps the full colonels of said regiments (they came and went too), and could vary from year to year, assembly to assembly, etc.
I would suggest that following John the OFM's advice would be the safest course of action though, and no one could really prove you wrong, as unfortunately these colours didn't seem to survive for us to see today, and I don't think there are any period drawings or paintings of them either.
While these units sometimes fielded as complete battalions, they were seldom used in combat as a combined force. The British generals tended to use them for garrison duty, road building, guarding supplies, etc. A quick look at the casualty rate of the provincial regiments at Fort Ti in 1758, as compared to the British regiments there, would tend to support this idea. Some provincial units on the Forbes campaign did indeed see service, but more as companies than battalions – at least in combat anyway.
Note that in the biggest field battles at Quebec there were no provincial units at all.
If you are doing a local skirmish type gaming, then militia or an individual company of provincials would be more historically correct, and the fort would probably have flown an English naval type ensign of a red field, which seemed to be used for forts of the period.