E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
What is delivered. The clothes of a man-servant delivered to him by his master. The stables to which your horse is delivered for keep. During the Merovingian and Carlovingian dynasties, splendid dresses were given to all the members of the royal household; barons and knights gave uniforms to their retainers, and even a dukes son, serving as a page, was clothed in the livery of the prince he served. (French, livrer.)
What livery is we know well enough; it is the allowance of horse-meate to keepe horses at livery; the which word, I guess, is derived of delivering forth their nightly food.Spenser on Ireland.
Livery. The colours of a livery should be those of the field and principal charge of the armorial shield; hence the Queens livery is gules (scarlet) or scarlet trimmed with gold. The Irish regiments preserve the charge of their own nation. Thus the Royal Irish Dragoon Guards have scarlet uniform with blue facings, and the Royal Irish Lancers have blue uniform with scarlet facings.