Think of two layers, one running quietly beneath the other. On top is what everyone notices — the words said out loud, the colour you see, the polite sentence in the email. Underneath there is a second layer that never announces itself but changes everything above it. An undertone is always that lower layer: a voice kept below the room's volume, a feeling travelling under the words, a colour sitting under the colour.
From the word’s history
The supplied source builds the word from under- plus tone. Since tone in English covers a sound, a way of speaking and a shade of colour, the compound reaches all three of its modern uses without changing shape.
來源資料把這個字拆成 under-（在下面）加上 tone。tone 在英文裡本來就同時指聲音、語氣和色調，所以這個組合一次就涵蓋了今天的三種用法：底下的聲音、底下的語氣、底下的顏色。字面意思一直沒有變，變的只是「底下那層」是什麼東西。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:881579430): ‘From under- + tone.’
登入就可以看每一支怎麼連回核心畫面。