Today I saw an advert for a diet food including ‘five blacks’ (itsutsu no kuro) and there are plenty of juices boasting how many purple things are in them. Although generalising the health properties of foods by colour is certainly not unique to Japan, it seems to be more common and more discussed than I remember back home.
I wonder if it all started with the more traditional health drink of ao jiru (green juice- or literally blue juice – made from various green leaves including kale) and spread from there colour by colour in a typical Japanese process of innovations through variations.