Orthodox, Pious, Righteous. The founders of the Empire.
Highguard characters live in the central-eastern Empire, in units called "chapters". Their aesthetic is very churchey - think clerics, paladins and crusaders. Military service is a big part of Highguard culture, so a military aesthetic is very prevalent regardless of the type of character you're playing.
They aren't celibate or prudish, but they do wear veils and hoods for moments of significance.
Greyscale with Bright or Metallic Accents
Highguard tend to look very "severe", blocks of black, white and greys with bright accents and metallics, often in a single colour to match your Chapter insignia. Most Highguard chapters have a signature colour to give them a uniform, unit appearance.