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.
Bit of an unusual cut for larp - but I could see this working as part of an outfit for a Kallavesi mystic or priest, or even a Highguard character that wants to convey a bit of a stern, businesslike impression.
Velvet Robe with Lace Trim
A stretch velvet robe with lace trim. Available in maroon and black, would seem at home in Highguard, but also potentially as a Temeschwari League or Varushka layer.
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
Plain Blue Tunic
Decent starting point for Steinr - easily modified with a bit of garment trim for less than a fiver and half an hour's work if you wanted to upgrade. Also potentially good for Kallavesi, or even Dawnish and Highguard characters.
"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.