There is a book on a table at the Assembly Hall of the Guard in Serpents Hold. It talks about Guard conduct, uniform, etc. I guess at each event members are awarded merits as well as demerits based on their actions such as following the commanders orders, wearing uniform, how you conduct yourself around your fellow guard, etc.. Based on this system of merits/demerits you are either rewarded with promotion or demoted within the guard.
Originally there were I think 4 classes for the guard if I remember correctly (Spell casters, Fighters, Tamers, Archers). Around Serpents hold you will find their corresponding buildings (bases) with examples of the uniforms they wear. I think there are generally 3 displayed representing lower to higher rank.
Everyone starts as a private I believe with the basic uniform. I assume promotions lead ya to the next rank and uniform.
Its been a while since I've attended events and then there is the whole blackrock addiction thing and hearing voices and something about burnt villages, but I digress. So it might be better to simply ask Commander Gwen for a more accurate explanation.