undercurrent vs undertone

undercurrent and undertone both are nouns.

Word	Noun	Adjective	Verb	Adverb
undercurrent	Yes	No	No	No
undertone	Yes	No	No	No

As nouns, undercurrent and undertone are synonyms defined as:

undercurrent and undertone: a subdued emotional quality underlying an utterance; implicit meaning

undercurrent (noun)	undertone (noun)
a subdued emotional quality underlying an utterance; implicit meaning	a pale or subdued color
a current below the surface of a fluid	a subdued emotional quality underlying an utterance; implicit meaning
	a quiet or hushed tone of voice