House Gaurds wear surcoats, not tabards. Bear in mind that what you have seen so far is usually ceremonial armour. You should recall that the armour usually worn by Houseguards, while on Agamar, is matt grey or black. There are a few inconsistancies, which usually arise from the developing mental image I have. I finally have that image fixed.
Allow me to explain, at great length, since the world building thread has siezed up:
Agamar has a class system based on the amount of money/property a person has. Usually this refers only to men as it is directly linked to military service and pitched battles are the preserve of men. Women have other duties.
Ranks within the system (Provisional):
All Nobles are members of Noble Orders, that is they serve as armoured warriors on the battlefield and leaders of Legions and Armies.
The Earl who can unite the squabling factions on Agamar would be called "Overlord" or King. He would be a man with the support of both the nobles and the common men. Such a man has not existed in over a thousand years. Any man who would wish to be Overlord now must retrieve and wield the Kings Sword, the first longsword forged by Agamarian smiths and wielded by Garan Ser-Flax, first King of Agamar.
1/2. Earl/High Earl
These are men of extreme wealth and power, the Houseguards are the exclusive right of the Earls to raise and maintain. High Earls are those Earls who hold a practical advantage over their peers and as a result are elevated above them. Earls conrol Earldoms, High Earls control the loyalty of the other Earls around them and form a High Earldom.
3/4. Lord/High Lord
These are men of wealth and power, the raising of soldiers, Legions and War Levies is their right. High Lords hold advantage over their peers and as a result are elevated above them, they collect tithes on behalf of their Earl.
These men are those wealthy enough to not only aford a war horse but also full armour and weapons. They are local majistrates and leaders of a Lord's Legions.
All nobles are equipped in the same way. The are armoured in a coat of ribbed metal scales laid over a base of chainmail, they whear greves to protect their shins and gauntlets to protect their hands and forarms. The helemt is made of two seperate halves rivited together with a crossbrace running from ear to ear; a neck guard is rivited to this and cheek plates that extend to protect the sides of the neck are hinged to the sides, another plate coves the face, the edges of which meet the edges of the cheek gaurds. A molded section in the face plate protects the nose with holes in the bottom allow him to breath, another hole cut over the mouth allows ease of speech. This helmet is lined with felt and is form fitting it weighs less than a great helm and allows greater visibility. The neck is protected by a pair of plates, front and back, which protects the throat and neck.
A noble's horse is of the finest quality and is protect by chainmail on its neck as well as plates protecting its chest and lower fore and hind legs. A coat of mail proctects the horses body.
The weapons of a noble are a ten foot lance-spear, a long sword, bastard sword or battle axe, a short sword and often a short bow or quiver of darts.
Agamarian warrior nobles' armour is blackened to prevent it from shining and they wear non descript coaks of a green of brown. High Earls may wear bright surcoats to inspire their troops through their own bravery but they usually die as soon as the yoemen farmers come within bow shot.
Agamarian shields are decorated in muted colours such as black, dark blue or green. House symbols are worn on etched on the gauntles and throat plates of the armour the etchings are filled with bronze which is dulled like the armour but remains visible at close quarters. Shields show the mark of the Earl a noble supports.
Equipped in this way Agamarian nobles are a devastating medium-heavy cavalry and a versatile battlefield force. The best warriors roam the field in small packs, independant of the highly diciplined field army, they are the Battledogs and can be recognized the the pelts they wear, taken from the Shadow Wolves who inhabit the darkest forest of the planet.
If you are interested I can post the rest of the class system but the nobles illustrate the point. The Longbowmen are the dominating soldiers on the battlefield and being visible will get you killed by a master bowman, of which there are many.
Additionally fancy uniforms are seen as a sign of arogance, for reasons which again relate to battlefield realities. Regular soldiers wear more colourful uniforms, relativly speaking but they're still not bright. Rank is determined by insignia on cloaks and armbands, which are usually something dark and black.
This is exclusive to Agamar. I have another planet, Arkamon, where all the nobles wear full plate trimmed with gold, surcoats and have shields with full hereldic coats of arms.
When the group goes to Tarsis Hal will remark on the high visibility of the Aesirian armour.