壓在底下的聲音
最字面的一層：聲音真的放在別人的音量底下。
noun · C1
A quiet, low voice. Almost always in the phrase in an undertone, or in undertones, when someone speaks softly so that only the people close by can hear.
低聲、壓低的聲音
核心畫面
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.
可以想成兩層東西，一層安靜地走在另一層底下。上面那層是大家都注意到的：說出口的話、看得見的顏色、信裡那句客氣的問候。…更多收起
可以想成兩層東西，一層安靜地走在另一層底下。上面那層是大家都注意到的：說出口的話、看得見的顏色、信裡那句客氣的問候。底下還有一層，從來不表明自己，卻會改變上面那層給人的感覺。undertone 指的一直都是下面那層。它可能是壓在全場音量底下的說話聲，可能是走在字面底下的情緒，也可能是藏在表面顏色底下的那一點顏色。三種用法畫面相同，差別只在那一層是聲音、是感覺，還是顏色。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.’
Three branches
走在話底下的情緒
把「底下那層」從音量換成情緒：字面上是一回事，底下另有東西在走。
noun · C1
A feeling or quality that words, a piece of writing, a film, an advertisement or a plan carries underneath what is actually said, so that you can sense it even though nobody states it.
言外之意、弦外之音
藏在顏色底下的顏色
同一層結構換成視覺：上面那個顏色是你看到的，底下那個決定它偏暖還是偏冷。
noun · C1
A pale colour sitting under the main colour you see, which quietly changes how that colour looks — for example skin with a warm yellow undertone, or grey paint with a blue undertone.
底色