1.  undertone(n = noun.communication) - a quiet or hushed tone of voice; "spoke in undertones"
is a kind of tone, tone of voice

2.  undertone(n = noun.cognition) undercurrent - a subdued emotional quality underlying an utterance; implicit meaning;
is a kind of meaning, substance

3.  undertone(n = noun.attribute) tinge - a pale or subdued color;
is a kind of shade, tincture, tint, tone

   A low or subdued tone or utterance; a tone less loud than usual. [1913 Webster]

undertone, n.
1 a subdued tone of sound or colour.
2 an underlying quality.
3 an undercurrent of feeling.

N  faintness, faint sound, whisper, breath, undertone, underbreath, murmur, hum, susurration, tinkle, still small voice, hoarseness, raucity, inaudible, scarcely audible, just audible, low, dull, stifled, muffled, hoarse, husky, gentle, soft, faint, floating, purling, flowing, whispered, liquid, soothing, dulcet, susurrant, susurrous, in a whisper, with bated breath, sotto voce, between the teeth, aside, piano, pianissimo, d la sourdine, out of earshot inaudibly, (ii) SPECIFIC SOUNDS.