All 5 Uses

undertone
in
The Blind Assassin
(Edited)

He puts his jacket around her shoulders. Old tweed, old tobacco, a singed odour. An undertone of salt.
p. 20.9

undertone = subtle trace
Her long hair is blonde, as mine was then — pale, white almost, as if the red undertones had been washed away — the iron, the copper, all the hard metals.
p. 46.1

undertones = subtle colors
Would there be no Chase and Sons? ... This must have been the reproachful undertone of more than one discussion, around the dinner table, over the port.
p. 65.1 *

undertone = underlying concern

editor's notes: Reproachful is a synonym for disapproving.
As I stood behind the table, Reenie in an undertone retailed the latest news.
p. 173.9 *

undertone = quiet voice
"You shouldn't pick on those weaker than yourself," said Callie in a pleased undertone.
p. 186.7

Definitions:

(1) (undertone as in: a subtle undertone)
a subtle characteristic

(a subtle characteristic is less obvious than surrounding characteristics -- though not necessarily less important)
(2) (undertone as in: said in an undertone)
a very quiet or hushed voice
(3) (meaning too rare to warrant focus)