Colour means measurement

The application is monochrome ink on paper. Almost nothing in the chrome is coloured: not the primary button, not the active nav item, not the brand.

Application chrome is monochrome ink on paper. The only chromatic events in the interface are measured attention.

The reason is not restraint for its own sake. In a product whose entire output is a reading of attention, colour is the strongest signal available for saying "this is the measurement". Spend it on a call-to-action and it stops meaning anything: the eye learns that colour is decoration and the charts lose their loudest voice.

It holds as a rule rather than a preference, which is what makes it survive. The dwell ramp is never allowed on a button. The chart palette is assigned in a fixed order, because that ordering is the thing that keeps adjacent series distinguishable to a colourblind reader — picking hues by eye would quietly break it.