The 1766 Chapter of Compact ruled that the aprons should be "indented with crimson with a visible T-over-H properly displayed as well as the indented Ribbon or Sash of this Order". The robes worn by the Three Principals are traditional, and not of any specific period of time, and they are coming from the ancient and world-wide custom of persons in authority such as judges, priests, scholastics, etc. The colours of the Royal Arch, 'blue, purple and scarlet' are taken from Exodus Ch25 V4 and Ch26 V1 but there have been some variations with time.
The surplices of white linen worn by the Sojourners date back from at least 1778 when the first Grand Chapter first approved them. The Scribe's surplice comes from the same period and is derived from the alb, a longer linen vestment from Greek and Latin origin worn by Christian priests since the third century AD. It symbolises the renewal of man in justice and in the holiness of truth.
The headdress was once part of the clothing of the Grand Principal as described in the 1796 rules of the Grand Chapter; this custom survives in many Chapters in the USA. Many other ordinary Chapters used also headdresses, but today it is rarely seen in English Chapters. The headdress was not originally a mitre in the style of those worn by bishops.
In the early chapter the sash was sometime worn on the right shoulder because they believed that, originally, the sash was a sword-belt that had to be this way to allow the sword to be easily grasped by the right hand. They, of course, had in mind the ancient Craftsmen who rebuilt the Temple with the sword at their side, and the trowel in hand. If this tradition is accepted, then we are dealing with a knightly degree probably of French origin. There is also a strong suspicion that those wearing the sash on their right shoulder were "Moderns" Masons, while the "Antients" wore it on the left one.
English Grand Chapter regulations of today require all Companions to wear a crimson-and-purple indented sash over the left shoulder, passing obliquely to the right, with silk fringe at the end, the emblem to be embroidered on a white background.
In the aprons of Past and Present Principals the fringe is of gold or metal gilt, and the emblem is on a crimson background. Grand Officers and Grand Superintendents and all the Companions of senior rank wear the same aprons as the Principals of Chapters, save that the emblem is on a dark blue background.
Certain officers (for instance the Grand Officers) wore collars, or chains, for a long period of time. During the last century the Grand Superintendents also wore collars and chains, and the privilege is now extended to many more officers. The Royal Arch Jewell may be worn in a Craft lodge but not the Royal Arch collar or chain. Collars, sashes and aprons belonging to Royal Arch Masonry cannot be worn in public.
© The Books by Gilles C. H. Nullens
Source material ...
Part B: Official History of Freemasonry - B.5.7 Royal Arch Clothings
The Books by Gilles C. H. Nullens - Wednesday, 31 March 2010