I can't take any credit and I hope 'Chad' doesn't mind but I've copied this almost verbatim from the Pendraken 10mm forum (there are some extremely knowledgeable people there for the FPW War):
Chad
Re: Zouave fanions
« Reply #6 on: 14 June 2014, 10:41:15 AM »
Found this on the t'internet.
Such a flag is not a standard as such, in the same sense as a regimental colour, but a fanion (the nearest English equivalent is perhaps battalion or company marker). It is used to mark the position of the unit, especially its commanding officer, on parade and in the field. They are only small, and were attached to a small staff which was placed in the muzzle of a rifle. The regulation sizes were 50cm x 40cm for a battalion fanion, 40 x 30 for a company fanion, and 34 x 27 for a platoon fanion (the latter can also be a triangular pennant 30 x 40).
The regulations of 1857 laid down the colours:-
1st battalion of each regiment: blue sheet
2nd battalion: red sheet
3rd battalion: yellow sheet
4th battalion: green sheet
When wartime expansion created extra battalions, these used fanions in the same sequence of colours, but with a central vertical white band down the sheet.
The devices were coloured by company (note that in the French army, companies were numbered consecutively through the regiment, and not the battalion):
1st, 5th, 9th, 13th: blue
2nd, 6th, 10th, 14th: red
3rd, 7th, 11th, 15th: yellow
4th, 8th, 12th, 16th: green
Chad