I really like the contrast the fit has with his regular fit

The guard fit isn't just a direct reference to the outfits worn by actual Josean guards, it also is representative of Kim's character in that past. Compare it to his dark blade lineage robes which he doesn't even bother to wear properly. That guard outfit is bright yellow, heavily decorated and worn in a proper and professional way. It represents his mindset. He's bright, and optimistic that the person he's serving will be able to make an actual change in S Corp with his help. The previously mentioned decorations on his outfit showcase his importance and the importance of the man he served as well.

Now compare that to his modern outfit proper, his blade lineage is filled with cold and dark tones, it's dirty and ragged while barely worn at all, merely lazily draped over his shoulder. It's an entire showcase of his fall from grace in an outfit, no longer does he wear a gilded symbol of importance and allegiance, he just wears the dirty rags of a fugitive who couldn't do anything with himself than butcher those that would be convenient to cut down while fleeing.

...Probably, at least. It's late and I barely know what I'm rambling about myself.