Servius mentions the toga trabea, which was woven in purple and saffron stripes, and worn by augurs.
The Fetiales were college of Roman priests who acted as the guardians of the public faith, It appears that when early Romans felt themselves aggrieved by a neighbouring city, four fetiales were deputized to seek redress. In turn, they elected one of their number to act as their representative. This person, called pater patratus populi Romani, wore a fillet of white wool around his head, together with a wreath of sacred herbs gathered within the enclosure of the Capitoline hill (verbenae, sagmina).
According to Pliny, the twelve Arval Bretheren, fratres arvales, wore, as badge of office, a chaplet of ears of corn (spicea corona) fastened on their heads with a white band.
There were more restrictive modes of dress and behaviour for those priests devoted to the worship of one god: the Flamen Dialis for Jove, the Flamen Martialis for Mars, and the Flamen Quirinalis for Romulus. (There were a host of minor flamines, but these were the most important, the flamines maior, and they were always chosen from the ranks of patricians.)
Their characteristic dress was the apex (a conical cap), the laena and a laurel wreath. The name, according to Varro and Festus, was derived from the band of white wool (filum, filamen, flaxmen) which was wrapped round the apex, and which they wore, without the apex, when the heat was oppressive.
Cicero has this to say about the Flamen Dialis, the most important of the flamines maior:
A great many ceremonies are imposed upon the Flamen Dialis, and also many restraints, about which we read in the books On The Public Priesthoods and also in Book I of Fabius Pictor's work. Among them I recall the following:
I) It is forbidden the Flamen Dialis to ride a horse;
II) It is likewise forbidden him to view the classes arrayed outside the pomerium [the sacred boundary of Rome], i.e., armed and in battle order ? hence only rarely is the Flamen Dialis made a Consul, since the conduct of wars is entrusted to the Consuls;
III It is likewise forbidden for him ever to take an oath by Jupiter;
IV Iit is likewise forbidden for him to wear a ring, unless it is cut through and empty;
V It is also forbidden to carry out fire from the flaminia, i.e., the Flamen Dialis' house, except for a sacral purpose;
VI if a prisoner in chains enters the house he must be released and the chains must be carried up through the opening in the roof above the atrium or living room onto the roof tiles and dropped down from there into the street;
VII He must have no knot in his head gear or in his girdle or in any other part of his attire;
VIII If anyone is being led away to be flogged and falls at his feet as a suppliant, it is forbidden to flog him that day;
IX The hair of the Flamen Dialis is not to be cut, except by a freeman;
X It is customary for the Flamen neither to touch nor even to name a female goat, or raw meat, ivy, or beans;
XI He must not walk under a trellis for vines;
XII The feet of the bed on which he lies must have a thin coating of clay, and he must not be away from this bed for three successive nights, nor is it lawful for anyone else to sleep in this bed;
XIII At the foot of his bed there must be a box containing a little pile of sacrificial cakes;
XIV The nail trimmings and hair of the Dialis must be buried in the ground beneath a healthy tree;
XV Every day is a holy day for the Dialis;
XVI He must not go outdoors without a head-covering ? this is now allowed indoors, but only recently by decree of the pontiffs, as Masurius Sabinus has stated; it is also said that some of the other ceremonies have been remitted and cancelled;
XVII It is not lawful for him to touch bread made with yeast;
XVIII His underwear cannot be taken off except in covered places, lest he appear nude under the open sky, which is the same as under the eye of Jove;
XIX No one else outranks him in the seating at a banquet except the Rex Sacrorum;
XX If he loses his wife, he must resign his office;
XXI His marriage cannot be dissolved except by death;
XXII He never enters a burying ground, he never touches a corpse ? he is, however, permitted to attend a funeral.
Almost the same ceremonial rules belong to the Flaminica Dialis, his wife. They say that she observes certain other and different ones, for example, that she wears a dyed gown, and that she has a twig from a fruitful tree tucked in her veil, and that it is forbidden for her to ascend more than three rungs of a ladder and even that when she goes to the Argei Festival [when twenty-four puppets were thrown into the Tiber] she must neither comb her head nor arrange her hair.
Source: Lacus Curtius: The Roman Religion, Articles of Smith's Dictionary: