I've seen this UI design pattern in a couple of places recently: colors overflowing the container shape. Is there a name for this pattern?

there is lib called blocks.css

Ah nice!

This in inspired from a pattern in offset printing where the alignment is off among the colour “screens”. That is why the “bleed” is symmetrical around edges. This was actually an unacceptable output in physical print. This is also paired usually with pastel and paper like colours that make digital interfaces feel tactile and friendly.