  $\require{mediawiki-texvc}$The green valley (GV) represents an important
transitional state from actively star-forming galaxies to passively evolving
systems. Its traditional definition, based on colour, rests on a number of
assumptions that can be subject to non-trivial systematics. In Angthopo et al.
(2019), we proposed a new definition of the GV based on the 4000$\AA$ break
strength. In this paper, we explore in detail the properties of the underlying
stellar populations by use of ~230 thousand high-quality spectra from the Sloan
Digital Sky Survey (SDSS), contrasting our results with a traditional approach
via dust-corrected colours. We explore high quality stacked SDSS spectra, and
find a population trend that suggests a substantial difference between low- and
high-mass galaxies, with the former featuring younger populations with star
formation quenching, and the latter showing older (post-quenching) populations
that include rejuvenation events. Subtle but measurable differences are found
between a colour-based approach and our definition, especially as our selection
of GV galaxies produces a cleaner "stratification" of the GV, with more
homogeneous population properties within sections of the GV. Our definition
based on 4000$\AA$ break strength gives a clean representation of the
transition to quiescence, easily measurable in the upcoming and future
spectroscopic surveys.
