Ok, this was brought up a while ago, but I ignored it as I was going back and forth on a couple of the choices.
The default Agency uniform is black and white – however, as a One Uniform policy would lead to the tropey “men in black” look, and indeed make them harder to identify for people who need to spot them (fae) in amongst the other random business people, it was long ago decided that each Agency uniform also requires a feature colour.
It also means that Solstice who move between areas are less likely to immediately spot Agency people as they would have it ingrained to (for example) look for “blue vest” rather than “black vest, blue highlights” or whatever).
Now, as there are only so many colours, a few major colours have been chosen, then the Agencies of the area use either the feature in a different way, or use a different shade.

Agency Feature Colours

Oceania – Blue – for, um, the ocean. Yes, there is a lot of jealously amongst Agencies that Oceania gets blue, when it’s felt that blue should belong to the Agency as a whole.
The Americas – white and red – the official feature colour is red, however there are many Agencies in the US that have moved away from including red, essentially having white as the default feature colour. This was done in a deliberate way to invoke the “men in black” image. For South America and Canada though, there is the usual percentages of high-use/low-use of the feature colour.
Middle-east – black – this is essentially the opposite of the policy above – with white being a usual predominant colour, leaving black as a feature colour.
Asia – Green – for jade. There was the option for yellow, due to the association with royalty (and then the further connection that Asia holds the Court of Kings) however…yeah, I’m not going to use “yellow” for Asia. -_-
Europe – metallic silver and purple – metallic silver is the primary colour, with purple generally being the highlight colour – this is both as a callback to knights in armour (remember, to this day, the French are the dragon hunters) and the purple as a callback to grapes/Greece and the influence on the modern world.
The Russian Exception – Whatever Grigori feels like wearing. His recruits use standard training outfits, but tend to avoid uniforms in public.
Africa – Yellow – inspired by the sun, as this is important healdry/symbolism for the Court of Life, whose seat is here.
Antarctica – rainbow – the lone primary agent here has a rainbow as his feature colour, as do the Court of Desolation reps who use his Agency as a secondary work area.