Article References:
AUXILIARY SHIELD PATTERNS
The latin term, clipeus is often used by modern historians to describe the flat oval shield carried by the Roman auxiliaries. However, this is not appropriate. The Roman applied the name clipeus to circular shields such as those carried by the ancient Greeks. Scutum was the general latin word for "shield," but there does not appear to be a specific word for the oval shields used by the auxilia.
The one sculpture that depcits the greatest number of shield design is Trajan's Column. This work was created at the very hight of the Roman Empire and its army. The shields on the column represent a very specific time in Roman history, but their symbolism could be applied to anytime in early to mid imperial history. Lino Rossi, a modern historian, conveyed his ideas on the symbolism in his now out of print book, Trajan's Column and the Dacian Wars. I present a streamlined version of his concept here:
Numbers 1-4 were probably units granted Roman citizenship for valor. 4-8 were units raised from Roman citizen volunteers.
9-18 Are shields probably from units that were awarded the title torquata.
Various auxiliary designs
The wings and thunderbolt design was almost never seen on the non-citizen auxiliary shields. The few times that it did appear could be explained by attributing those instance to auxiliaries who were awarded citizenship or those units who were not citizens, but served the emperor directly. (such as the emperor's bodyguard)
The wreath design was common on auxiliary shields. A laurel wreath (or cown) was awarded to a general who retuned from battle with victory. It can be guessed that whe this design was painted on a shield it meant that particular unit had recieved an award for some great victory.
Color Reconstructions:
It would seem that auxiliary shield designs were more ornamental than symbolic. The most prevelant design among the auxiliaries was the wreath. As explained above, this design most probably meant that the unit was victorious. Many of the other patterns, however, were merely simple shapes such as stars, circles, flowers, etc.
References:
1. Greece and Rome at War, by Peter Connolly.
2. Rossi, Lino Trajan's Column and the Dacian Wars.