The Collaborative International Dictionary of English v.0.48:

Undertone \Un"der*tone`\, n.
   A low or subdued tone or utterance; a tone less loud than
   usual.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "undertone":
   affective meaning, air, allegory, allusion, arcane meaning,
   aspiration, assumption, atmosphere, aura, bated breath, bearing,
   breath, breathy voice, climate, coloration, coloring, connotation,
   consequence, denotation, drift, effect, essence, exhalation,
   extension, feel, feeling, force, gist, grammatical meaning, hint,
   idea, impact, implication, implied meaning, import, inference,
   innuendo, intension, intimation, ironic suggestion,
   lexical meaning, literal meaning, little voice, low voice, meaning,
   metaphorical sense, milieu, mumble, mumbling, murmur, murmuration,
   murmuring, mutter, muttering, note, nuance, occult meaning,
   overtone, pertinence, pith, point, practical consequence,
   presumption, presupposition, purport, quality, range of meaning,
   real meaning, reference, referent, relation, relevance, rumor,
   scope, semantic cluster, semantic field, sense, sigh, significance,
   signification, significatum, signifie, soft voice, span of meaning,
   spirit, stage whisper, still small voice, structural meaning,
   subsense, subsidiary sense, substance, suggestion, sum,
   sum and substance, supposition, susurration, susurrus,
   symbolic meaning, symbolism, tenor, tinge, tone,
   totality of associations, touch, transferred meaning,
   unadorned meaning, underbreath, undercurrent, undermeaning, value,
   whisper, whispering