Elementary colors. It is not a reaction from the three elementary colors, but impressions given by objects.
It is a decoration hidden in the story, as the names of characters insinuate certain colors.
To describe it from a more difficult to understand perspective: the colorless Shiki, the transparent Kirie.1

Translator’s Notes
  1. ^ Not exactly your straight forward wordplay, but perhaps that was the intention.
Garden of sinners Pamphlet: Kara no Kyoukai Settings Glossary