a low or subdued tone

to speak in undertones.

an unobtrusive or background sound

an undertone of static from the receiver.

an underlying quality or element; undercurrent

There was an undertone of regret in his refusal.

a subdued color; a color modified by an underlying color.

noun

association , atmosphere , buzz , connotation , feeling , flavor , hint , hum , implication , low tone , mumble , murmur , mutter , overtone , rumor , tinge , touch , trace , undercurrent , inkling , suspicion , aura , mood , sense , suggestion , whisper