Greys are used to create the core building blocks - banding, tiles, etc. - which allows colour to be added to give consistent and specific meaning.
Salem is used for primary actions e.g. search, primary call to action buttons. Poppy is used to highlight errors, e.g. validation errors on forms. Carrot is used for global messages e.g alpha/beta banners or messages. Text links are Matisse.
These colours are used for census branding or/and pages.