Agree with your comments on the sad state of Marlborough's trophies (having said that there are flags from the Seven Years War which are still around, but they are not British), and that it is a guessing game for a lot of these units. But I think you need to take a look at the photo (because it is a photo of an actual Mitre which you can go and see, not a drawing, and is from the National Army Museum), also I have other sources which also say that the 18th Royal Irish were the only other unit known to wear the stiff mitre during this period. If I am correct it comes from the memoirs of Captain Robert Parker who was a captain of the grenadier Company in the Royal Irish regement during the WSS. When George I came to the throne in 1715, one of the symbols of the House of Hanover was the white horse on a red field(there were 2 exceptions a guards regement (had a blue background) and a Welsch regement (had a sheep instead of a horse, typical of the Welsch). All Grenadier Companies had this on their headdress so the photo of the mitre predates this as it does not.
Iain Stamford probably knows more about it than either of us, and he and I have discussed uniform details about the 1st Orkneys and 23rd Royal Welsch Fusiliers, Royal Scots Fusiliers(21st) and artillary. If you are a member of the Pike & Shot Yahoo user group you can see our conversations there, they make interesting reading. Iain's view on the 1st Orkneys was that they had white and not blue facing for the entire period of the WSS and did not change to blue until George I came to the throne. (I noticed you have done Orkneys in blue, so have I). It is known that they were certainly white during the LOA but do not know when they changed to blue. My view is that they changed from white to blue in 1707 after the act of union between England & Scotland (which our Scottish Friends in the SNP are trying to dissolve) in line with the Scots Guards who changed from white to blue at this date, as they were also a "Royal" regiment. You are right about the act of union between Britain and Ireland happening in 1806 from which we get our current union flag. But from what I understand after 1707 the union flag was issued and carried in the field, but as with the Napelonic Wars, only after the old flags had worn out, so you will get a mish mash. The first units to carry new flags were generally the Guards (as they got them first). So I would imagine the Colstream Guards carried the post 1707 flag at the two WSS battles they were present at which were both post 1707.
Re the Scots Fusiliers having red cuff and not blue. Iain Stanford pointed out to me that since the fusiliers were raised to protect the artillary, they had the same uniform as the artillary. For the Scottish establishment this was red and for the English (and Welsch) establishment this was blue(there was also an Irish establishment, Britian in fact having 3 armies at this stage English, Scottish and Irish with a lot of the units going to Flanders coming from the Irish establishment), hence the Scot Fusiliers having red cuffs and the English and Welsch Fusiliers having blue cuffs. The Welsch fusiliers actually became fusiliers in 1702 being a conversion of an existing unit (later being numberd 23, interesting to note that a company from their 2nd battalion won all those VC's at Rourkes Drift against the Zulus).
But when faced with a dilema like this I always use the wargamers get out clause, They look really good painted in those colours, so I will ignore history, and your figures certainly do so why change them.
Thanks for the debate.