Following comments and emails received as a result of my little post on Spanish cavalry flags, here's a little more evidence. These are taken from Jose-Maria Bueno's Uniformes Militares Espanoles - El Exercito y la Armada en 1808.
I reproduce these without any permission - I own the book, and they are offered here for purposes of private study(!) - if anyone is offended or compromised please shout and I'll delete the post.
With apologies for my rather approximate grasp of Spanish, I translate part of the second excerpt thus:
By a Royal Ordnance of 1768 the regiments of cavalry of the guard, of dragoons and of line and light cavalry were to employ banners of damask or velvet of colour blue, red or white according to the regiment, with the royal arms embroidered in relief in gold or silver on the obverse, and the insignia of each regiment on the reverse. In the 4 corners, embroidered in gold or silver, fleurs-de-lys [Bourbon symbol].
Cravats for all standards and flags to be red, for this is the national symbol.
Spearheads, fringes, cords and braid of the pole were also to be gold and silver, according to the lace of each regiment. The lance or pole to be covered with crimson fabric or velvet.
These regulations lasted until 1833, so were still in force in the period of our study.
Bueno's book is obviously not the Bible, but he is regarded as a serious authority - especially by me. His work was published in 1982, which is a year or two after Terry and I were scratching around in the dark!
The text for Fig.156 suggests that the various squadrons of this regiment had different coloured flags - white for the first squadron, red for the remainder. All just clues, remember. Sadly, the passing years, countless generations of moths and the occasional political upheaval have removed most of the actual evidence.