Apparently it's a holiday souvenir of the Ginza line in the Tokyo metro.
https://twitter.com/fet/status/380463862165684224
I would personally add that this is generally a very good redesign, but I do wonder whether a circle thick enough to draw attention amidst all the noise and bustle of a busy metro station isn't a little too thick and attention-seeking for a small screen.
Particularly when there are multiple colored circles on a phone screen, that's fairly strong visual competition for the adjacent text, and seems to come a little adrift from Apple's principle of UI 'deference' to user data.
The metro signs seem a very reasonable and interesting source of inspiration, but I think that adaptation for context, medium, and cognitive efficiency may work better than decorative copy-paste. Slightly thinner and less obtrusive circles would work better for me.
(Perhaps others respond differently – I know some people like to be 'hit over the head' by bright colors to warn of impending due or elapsed due dates :-)
Generally, however, a really encouraging redesign, I think.
--