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.
Cool and made from IC looking fabric. They come in loads of colours, if you can only find pale ones try dying them a more on brief colour. You can get these from about March to September, from loads of different shops, but sometimes you get what you pay for and the cheaper ones might not last more than a year.
£10 - 25
"Meggings". Like leggings but y'know, Manly.
Don't worry about the waistband - your upper layer will hide it. A pair of comfy looking grey leggings that would be fine base layers for basically any nation, and specifically in a men's cut. I put them in my wintermark folder originally but whatever.