Source : Webster's Revised Unabridged Dictionary (1913)

Undertone \Un"der*tone`\, n.
   A low or subdued tone or utterance; a tone less loud than
   usual.

Source : WordNet®

undertone
     n 1: a quiet or hushed tone of voice; "spoke in undertones"
     2: subdued emotional quality underlying an utterance; implicit
        meaning [syn: {undercurrent}]
     3: a pale or subdued color [syn: {tinge}]