Harder than choosing colors is deciding their proportions. Lay five good colors out at equal size and you usually end up with no color at all.
One color per city is too narrow
Ask what color Seoul is and it's hard to answer. The dancheong of the palaces is red and blue, the dawn sky is navy, the subway lines are yellow and green, and spring blossom is pale pink.
Lock a city into one color and the object stops being true. So kory uses five colors per city. The problem is what comes next: how do you arrange five?
70 / 20 / 10
The rule we settled on is simple. Seventy percent for the base color, twenty for the primary that sits on top of it, and ten for the accent.
The 70 should not stand out. It is the surface you see first when you pick the thing up, and the one that has to stay easy on the eye. The 20 builds form on top of it. The 10 appears in the smallest amount — and it decides which city this object belongs to.
The 70 changes with the city
Seoul's 70 is the navy of dawn. Busan's is the deep blue of the daytime sea, Jeju's is volcanic black, and Korea's is the warm off-white left after a sunset.
Change the base and the same color on top looks entirely different. Orange on black and orange on off-white feel like different colors. That is why the pieces from each city never resemble one another.
It is the ten percent that builds the brand. The 70 keeps you looking; the 10 makes you remember.
The proportions get tested, again and again
This is a starting point more than a rule. Applied to a real object, the same proportion reads differently depending on the material. A color printed on fabric and the same value set into plastic are not the same color.
So we look again with every sample — judging not whether the numbers are right, but whether the city comes to mind when the thing is in your hand.