undertone vs undercurrent

undertone and undercurrent both are nouns.

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

As nouns, undertone and undercurrent are synonyms defined as:

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

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