Late 13thc. early 14thc. heraldry is still pretty simple. There is a book by Fox-Davies called Heraldry that has some wonderful examples of heraldic devices of this period. Secular Knighthood is loaded with colorful designs and livery that can make for a pretty colorful figure. The design painted on the shield was usually some element or badge from the bearer's coat of arms. Animals, body parts, celestial designs, mythical animals, geometric shapes, different types of geometric divisions of colors...This was a sort of early medieval uniform and soldiers recognized each other via the color surcoats and arms painted on shields and banners. A household knight may carry his Lord's heraldry or badge on his shield or if he was in charge of his own Lance, (a group of soldiers he brought to the field) then he may carry his own arms and his soldiers or levied trrops would carry anything from a simple one color shield or surcoat to a partied color shield (a color and metal usually white or yellow representing silver or gold) or a badge or device of their leaders arms.
The religous orders may carry several types of crosses dependent on those reprsentative of their order. i.e. white cross on black shield Order of St. John prior to 1270, white cross on red shield, Order of St. John after the papal bull of 1270. red cross on white field, Knight Templar, (shield may have been black and white representative of their banner) black cross on white field, teutonic order. Redcross with a sharpened lower point on a white field was one of the spanish orders, Santiago I think, A little research will get you more info.
The shields were made of wooden boards almost like a sheet of plywood, covered with a layer of horsehair and then leather. The back side was most likely painted any color if not covered with leather. Alot of the figures out there have shields with a wood grain on their backsides so if you wanted to you could paint it as such. (Shields were pretty much a disposable item and replaced between battles if damaged. Most of the extant shields in museums are funerary achievements and were preserved and not reliably representative of actual "combat" shields.
Hit the library and pull books on heraldry and you'll find a plethora of info.
Good luck.
Mike
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